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Abbot of Briinn 
Born 1822. Died 1884. 

From a photog7-aph kindly supplied by the Very Rev. Dr Janeischek, 
the p}-esent Abbot. 











TN the Study of Evolution progress had well- 
J- nigh stopped. The more vigorous, perhaps also 
the more prudent, had left this field of science 
to labour in others where the harvest is less pre- 
carious or the yield more immediate. Of those who 
remained some still struggled to push towards truth 
through the jungle of phenomena: most were content 
supinely to rest on the great clearing Darwin made 
long since. 

Such was our state when two years ago it was 
suddenly discovered that an unknown man, Gregor 
Johann Mendel, had, alone, and unheeded, broken oflP 
from the rest — in the moment that Darwin was at 
work — and cut a way through. 

This is no mere metaphor, it is simple fact. Each 
of us who now looks at his own patch of work sees 
Mendel's clue running through it : whither that clue 
\Yi\\ lead, we dare not yet surmise. 

It was a moment of rejoicing, and they who had 
heard the news hastened to spread them and take the 

vi Preface 

instant way. In this work I am proud to have borne 
my little part 

But every gospel must be preached to all alike. 
It will be heard by the Scribes, by the Pharisees, by 
Demetrius the Silversmith, and the rest. Not lightly 
do men let their occupation go ; small, then, would 
be our wonder, did we find the established prophet 
unconvinced. Yet, is it from misgiving that Mendel 
had the truth, or merely from indifference, that no 
naturalist of repute, save Professor Weldon, has risen 
against him ? 

In the world of knowledge we are accustomed to 
look for some strenuous effort to understand a new 
truth even in those who are indisposed to believe. 
It was therefore with a regret approaching to in- 
dignation that I read Professor Weldon's criticism*. 
Were such a piece from the hand of a junior it 
might safely be neglected ; but coming from Professor 
Weldon there was the danger — almost the certainty — 
that the small band of younger men who are thinking 
of research in this field would take it they had learnt 
the gist of Mendel, would imagine his teaching ex- 
posed by Professor Weldon, and look elsewhere for 
lines of work. 

In evolutionary studies we have no Areopagus. 
With us it is not — as happily it is with Chemistry, 

* Biometrika, i., 1902, Pt. ii. 

Preface vii 

Physics, Physiology, Pathology, and other well- 
followed sciences — that an open court is always 
sitting, composed of men themselves workers, keenly 
interested in every new thing, skilled and well versed 
in the facts. Where this is the case, doctrine is soon 
tried and the false trodden down. But in our sparse 
and apathetic community error mostly grows un- 
heeded, choking truth. That fate^ must not befall 
Mendel now. 

It seemed imperative that MendeFs own work 
should be immediately put into the hands of all who 
will read it, and I therefore sought and obtained the 
kind permission of the Royal Horticultural Society to 
reprint and modify the translation they had already 
caused to be made and published in their Journal. 
To this I add a translation of Mendel's minor paper 
of later date. As introduction to the subject, the 
same Society has authorized me to reprint with 
alterations a lecture on heredity delivered before 
them in 1900. For these privileges my warm thanks 
are due. The introduction thus supplied, composed 
originally for an audience not strictly scientific, is far 
too slight for the present purpose. A few pages are 
added, but I have no time to make it what it should 
be, and I must wait for another chance of treating 
the whole subject on a more extended scale. It wall 
perhaps serve to give the beginner the slight 

viii Preface 

assistance which will prepare him to get the most 
from Mendel's own memoir. 

The next step was at once to defend Mendel from 
Professor Weldon. That could only be done by 
following this critic from statement to statement in 
detail, pointing out exactly where he has gone wrong, 
what he has misunderstood, what omitted, what in- 
troduced in error. With such matters it is easy to 
deal, and they would be as nothing could we find in his 
treatment some word of allusion to the future ; some 
hint to the ignorant that this is a very big thing ; 
some suggestion of what it all may mean if it he 

Both to expose each error and to supply effectively 
what is wanting, within the limits of a brief article, 
written with the running pen, is difficult. For sim- 
plicity I have kept almost clear of reference to facts 
not directly connected with the text, and have foregone 
recital of the now long list of cases, both of plants 
and animals, where the Mendelian principles have 
already been perceived. These subjects are dealt 
with in a joint Report to the Evolution Committee of 
the Royal Society, made by Miss E. R. Saunders and 
myself, now in the Press. To Miss Saunders who 
has been associated with me in this work for several 
years I wish to express my great indebtedness. Much 

Prefam ix 

of the present article has indeed been written in 
consultation with her. The reader who seeks fuller 
statement of facts and conceptions is referred to the 
writings of other naturalists who have studied the 
phenomena at first hand (of which a bibliography is 
appended) and to our own Report. 

I take this opportunity of acknowledging the 
unique facilities generously granted me, as repre- 
sentative of the Evolution Committee, by Messrs 
Sutton and Sons of Reading, to watch some of the 
many experiments they have in progress, to inspect 
their admirable records, and to utilise these facts for 
the advancement of the science of heredity. My 
studies at Reading have been for the most part 
confined to plants other than those immediately the 
subject of this discussion, but some time ago I availed 
myself of a kind permission to examine their stock of 
peas, thus obtaining information which, with other 
facts since supplied, has greatly assisted me in treating 
this subject. 

I venture to express the conviction, that if the 
facts now before us are carefully studied, it will be- 
come evident that the experimental study of heredity, 
pursued on the lines Mendel has made possible, is 
second to no branch of science in the certainty and 
magnitude of the results it ofiers. Tliis study has 

X Preface 

one advantage which no other line of scientific inquiry 
possesses, in that the special training necessary for 
such work is easily learnt in the practice of it, and 
can be learnt in no other way. All that is needed is 
the faithful resolve to scamp nothing. 

If a tenth part of the labour and cost now devoted 
by leisured persons, in this country alone, to the 
collection and maintenance of species of animals and 
plants which have been collected a hundred times 
before, were applied to statistical experiments in 
heredity, the result in a few years would make a 
revolution not only in the industrial art of the breeder 
but in our views of heredity, species and variation. 
We have at last a brilliant method, and a solid basis 
from which to attack these problems, ofibring an 
opportunity to the pioneer such as occurs but seldom 
even in the history of modern science. 

We have been told of late, more than once, that 
Biology must become an exact science. The same is 
my own fervent hope. But exactness is not always 
attainable by numerical precision : there have been 
students of Nature, untrained in statistical nicety, 
whose instinct for truth yet saved them from perverse 
inference, from slovenly argument, and from misuse 
of authorities, reiterated and grotesque. 

The study of variation and heredity, in our ignor- 
ance of the causation of those phenomena, must be 

Preface xi 

built of statistical data, as Mendel knew long ago ; 
but, as he also perceived, the ground must be pre- 
pared by specific experiment. The phenomena of 
heredity and variation are specific, and give loose and 
deceptive answers to any but specific questions. That 
is where our exact science will begin. Otherwise we 
may one day see those huge foundations of "biometry '^ 
in ruins. 

But Professor Weldon, by coincidence a vehement 
preacher of precision, in his haste to annul this first 
positive achievement of the precise method, dispenses 
for the moment even with those unpretending forms 
of precision which conventional naturalists have use- 
fully practised. His essay is a strange symptom of 
our present state. The facts of variation and heredity 
are known to so few that anything passes for evidence ; 
and if only a statement, or especially a conclusion, be 
negative, neither surprise nor suspicion are aroused. 
An author dealing in this fashion with subjects com- 
monly studied, of which the literature is familiar and 
frequently verified, would meet with scant respect. 
The reader who has the patience to examine Professor 
Weldon's array of objections will find that almost all 
are dispelled by no more elaborate process than a 
reference to the original records. 

With sorrow I find such an article sent out to 
the world by a Journal bearing, in any association, 

xii Preface 

the revered name of Francis Galton, or under the 
high sponsorship of Karl Pearson. I yield to no one 
in admiration of the genius of these men. Never 
can we sufficiently regret that those great intellects 
were not trained in the profession of the naturalist. 

Mr Galton suggested that the new scientific firm 
should have a mathematician and a biologist as 
partners, and — soundest advice — a logician retained 
as consultant*. Biologist surely must one partner be, 
but it will never do to have him sleeping. In many 
well-regulated occupations there are persons known 
as " knockers-up," whose thankless task it is to rouse 
others from their slumber, and tell them work-time is 
come round again. That part I am venturing to play 
this morning, and if I have knocked a trifle loud, it is 
because there is need. 

March, 1902. 

Biometrilca, i. Pt. t. p. 5. 



The Problems of Heredity and their Solution, pp. 1 — 39. 

Preliminary statement of Mendel's principles, 8. Re- 
lation of Mendel's discovery to the law of Ancestral 
Heredity, 19. Heterozygote and Homozygote^ 23. New 
conceptions necessitated by Mendel's discovery, 26. Simple 
alternative characters, or allelomorphs, 27. Compound 
allelomorphs and their components, 29. Analytical Varia- 
tions, 29. Relation of Mendel's principle to continuous 
variation, 32. Dominance, 32. Non-Mendelian pheno- 
mena, 33. False hybrids of Millardet, 34. Brief historical 
notice, 36. 


pp. 40—95. 

Introductory Remarks, 40. Selection of Experimental 
Plants, 42. Division and Arrangement of Experiments, 44. 
Characters selected, 45. Number of first crosses, 47. 
Possible sources of error, 47. Forms of the Hybrids, 49. 
Dominant and recessive, 49. 

First generation bred from the Hybrids, 51. Numbers 
of each form in ofispring, 52. Second generation bred from 
the Hybrids, 55. Subsequent generations bred from the 
Hybrids, 57. 

Offspring of Hybrids in which several differentiating 
characters are associated, 59. The reproductive cells of 
the Hybrids, 66. Statement of Mendel's essential deduc- 
tions, 67. Experiments to determine constitution of germ- 
cells, 68. Statement of purity of germ-cells, 72. 

Experiments with Phaseolus, 76. Compound characters, 
80. Concluding Remarks, 84. 


xiv Contents 

HEREDITY, 104—208. 
Introductory, 104. 

I. The Mendelian Principle of Purity of Germ-cells 

AND THE Laws of Heredity based on Ancestry, 108. 

II. Mendel and the critic's version of him. 

The Law of Dominance, 117. 

III. The facts in regard to Dominance of Characters in 

Peas, 119. 
The normal characters : colours of cotyledons and seed- 
coats, 120. Shape, 122. Stability and variability, 124. 
Results of crossing in regard to seed-characters : normal and 
exceptional, 129. Analysis of exceptions, 132. The "mule" 
or heterozygote, 133. 

IV. Professor Weldon's collection of "Other evidence 

concerning Dominance in Peas." 

A. In regard to cotyledon colour: Preliminary, 137. 
Xenia, 139. (1) Gartner's cases, 141. (2) Seton's case, 143. 
(3) Tschermak's exceptions, 145. (3 a) Buchsbaum case, 145. 
(3 h) Telephone cases, 146. (3 c) Couturier cases, 147. 

B. Seed-coats and Shapes. 1. Seed-coats, 148. 2. Seed- 
shapes : (a) Rimpau's cases, 150. (6) Tschermak's cases, 152. 
3. Other phenomena, especially regarding seed-shapes, in 
the case of " grey " peas. Modern evidence, 153. 

C. Evidence of Knight and Laxton, 158. 

D. Miscellaneous cases in other plants and animals : 

1. Stocks {Matthiola). Hoariness, 169. Flower- 
colour, 170. 

2. Datura, 172. 

3. Colours of Rats and Mice, 173. 

V. Professor Weldon's quotations from Laxton, 178. 

Illustration from Primula sinensis, 182. 

VI. The argument built on exceptions, 183. 

Ancestry and Dominance, 185. 
Ancestry and purity of germ-cells, 193. 
The value of the appeal to Ancestry, 197. 

VII. The question of absolute purity of germ-cells, 201. 
Conclusion, 208. 


p. 22, par. 3, line 2, for " falls" read " fall." 

p. 63, line 12, for ''AabbC" read '' AaBbc.'' 

p. 66, in heading, for " of hybrids " read " of the hybrids." 

Note to p. 125. None of the yellow seeds produced by Laxton's 
Alpha germinated, though almost all the green seeds sown gave 
healthy plants. The same was found in the case of Express, another 
variety which bore some yellow seeds. In the case of Blue Peter, on 
the contrary, the yellow seeds have grown as well as the green ones. 
Few however were wholly yellow. Of nine yellow seeds produced by 
crossing green varieties together (p. 131), six did not germinate, 
and three which did gave weak and very backward plants. Taken 
together, this evidence makes it scarcely doubtful that the yellow colour 
in these cases was pathological, and almost certainly due to exposure 
after ripening. 

2 The Problems 

It is in the hope of inducing others to follow these 
lines of investigation that I take the problems of heredity 
as the subject of this lecture to the Eoyal Horticultural 

No one has better opportunities of pursuing such 
work than horticulturists and stock breeders. They are 
daily witnesses of the phenomena of heredity. Their 
success also depends largely on a knowledge of its laws, 
and obviously every increase in that knowledge is of 
direct and special importance to them. 

The want of systematic study of heredity is due 
chiefly to misapprehension. It is supposed that such 
work requires a lifetime. But though for adequate study 
of the complex phenomena of inheritance long periods 
of time must be necessary, yet in our present state of 
deep ignorance almost of the outline of the facts, obser- 
vations carefully planned and faithfully carried out for 
even a few years may produce results of great value. In 
fact, by far the most appreciable and definite additions 
to our knowledge of these matters have been thus 

There is besides some misapprehension as to the 
kind of knowledge which is especially wanted at this 
time, and as to the modes by which we may expect to 
obtain it. The present paper is written in the hope that 
it may in some degree help to clear the ground of these 
difiiculties by a preliminary consideration of the question, 
How far have we got towards an exact knowledge of 
heredity, and how can we get further? 

Now this is pre-eminently a subject in which we 
must distinguish what we can do from what we want 
to do. We want to know the whole truth of the matter ; 
we want to know the physical basis, the inward and 

of Heredity 3 

essential nature, "the causes," as they are sometimes 
called, of heredity: but we want also to know the laws 
which the outward and visible phenomena obey. 

Let us recognise from the outset that as to the essential 
nature of these phenomena we still know absolutely 
nothing. We have no glimmering of an idea as to what 
constitutes the essential process by which the likeness 
of the parent is transmitted to the offspring. We can 
study the processes of fertilisation and development in 
the finest detail which the microscope manifests to us, 
and we may fairly say that we have now a considerable 
grasp of the visible phenomena ; but of the nature of 
the physical basis of heredity we have no conception 
at all. No one has yet any suggestion, working hypo- 
thesis, or mental picture that has thus far helped in 
the slightest degTee to penetrate beyond what we see. 
The process is as utterly mysterious to us as a flash of 
lightning is to a savage. We do not know what is the 
essential agent in the transmission of parental characters, 
not even whether it is a material agent or not. Not only 
is our ignorance complete, but no one has the remotest 
idea how to set to work on that part of the problem. 
We are in the state in which the students of physical 
science were, in the period when it was open to anyone 
to believe that heat was a material substance or not, as 
he chose. 

But apart from any conception of the essential modes 
of transmission of characters, we can study the outward 
facts of the transmission. Here, if our knowledge is 
still very vague, we are at least beginning to see how 
we ought to go to work. Formerly naturalists were 
content with the collection of numbers of isolated instances 
of transmission — more especially, striking and peculiar 


4 The Problems 

cases — the sudden appearance of highly prepotent forms, 
and the like. We are now passing out of that stage. 
It is not that the interest of particular cases has in 
any way diminished — for such records will always have 
their value — but it has become likely that general ex- 
pressions will be found capable of sufficiently wide appli- 
cation to be justly called "laws " of heredity. That this 
is so was till recently due almost entirely to the work of 
Mr F. Galton, to whom we are indebted for the first 
systematic attempt to enuntiate such a law. 

All laws of heredity so far propounded are of a 
statistical character and have been obtained by statistical 
methods. If we consider for a moment what is actually 
meant by a "law of heredity" we shall see at once why 
these investigations must follow statistical methods. For 
a "law" of heredity is simply an attempt to declare 
the course of heredity under given conditions. But if 
we attempt to predicate the course of heredity we have 
to deal with conditions and groups of causes wholly 
unknown to us, whose presence we cannot recognize, 
and whose magnitude we cannot estimate in any par- 
ticular case. The course of heredity in particular cases 
therefore cannot be foreseen. 

Of the many factors which determine the degree 
to which a given character shall be present in a given 
individual only one is usually known to us, namely, 
the degree to which that character is present in the 
parents. It is common knowledge that there is not that 
close correspondence between parent and offspring whicH 
would result were this factor the only one operating ; 
but that, on the contrary, the resemblance between the 
two is only an uncertain one. 

In dealing with phenomena of this class the study 

of Heredity 5 

of single instances reveals no regularity. It is only by 
collection of facts in great numbers, and by statistical 
treatment of the mass, that any order or law can be 
perceived. In the case of a chemical reaction, for instance, 
by suitable means the conditions can be accurately repro- 
duced, so that in every individual case we can predict 
with certainty that the same result will occur. But with 
heredity it is somewhat as it is in the case of the rainfall. 
No one can say how much rain will fall to-morrow in 
a given place, but we can predict with moderate accuracy 
how much will fall next year, and for a period of years 
a prediction can be made which accords very closely with 
the truth. 

Similar predictions can from statistical data be made as 
to the duration of life and a gTeat variety of events, the 
conditioning causes of which are very imperfectly under- 
stood. It is predictions of this kind that the study of 
heredity is beginning to make possible, and in that sense 
laws of heredity can be perceived. 

We are as far as ever from knowing why some characters 
are transmitted, while others are not ; nor can anyone yet 
foretell which individual parent will transmit characters to 
the offspring, and which will not ; nevertheless the progress 
made is distinct. 

As yet investigations of this kind have been made in 
only a few instances, the most notable being those of 
Galton on human stature, and on the transmission of 
colours in Basset hounds. In each of these cases he has 
shown that the expectation of inheritance is such that a 
simple arithmetical rule is approximately followed. The 
rule thus arrived at is that of the whole heritage of the 
offspring the two parents together on an average contribute 
one half, the four grandparents one-quarter, the eight 

6 The Problems 

great-grandparents one-eighth, and so on, the remainder 
being contributed by the remoter ancestors. 

Such a law is obviously of practical importance. In 
any case to which it applies we ought thus to be able to 
predict the degree with which the purity of a strain may 
be increased by selection in each successive generation. 

To take a perhaps impossibly crude example, if a 
seedling show any particular character which it is desired 
to fix, on the assumption that successive self-fertilisations 
are possible, according to Galton's law the expectation of 
purity should be in the first generation of self-fertilisation 
1 in 2, in the second generation 3 in 4, in the third 7 in 8, 
and so on"^. 

But already many cases are known to which the rule in 
any simple form will not apply. Galton points out that 
it takes no account of individual prepotencies. There are, 
besides, numerous cases in which on crossing two varieties 
the character of one variety almost always appears in each 
member of the first cross-bred generation. Examples of 
these will be familiar to those who have experience in such 
matters. The offspring of the Polled Angus cow and the 
Shorthorn bull is almost invariably polled or with very 
small loose "scurs." Seedlings raised by crossing Atropa 
helladonna with the yellow-fruited variety have without 
exception the blackish-purple fruits of the type. In several 
hairy species when a cross with a glabrous variety is made, 
the first cross-bred generation is altogether hairy t. 

Still more numerous are examples in which the characters 
of one variety very largely, though not exclusively, pre- 
dominate in the offspring. 

* See later. Galton gave a simple diagrammatic representation of 
his law in Nature, 1898, vol. lvii. p. 293. 

+ These we now recognize as examples of Mendelian ' dominance.' 

of Heredity 7 

These large classes of exceptions — to go no further — 
indicate that, as we might in any case expect, the principle 
is not of universal application, and will need various 
modifications if it is to be extended to more complex cases 
of inheritance of varietal characters. No more useful work 
can be imagined than a systematic determination of the 
precise "law of heredity" in numbers of particular cases. 

Until lately the work which Galton accomplished stood 
almost alone in this field, but quite recently remarkable 
additions to our knowledge of these questions have been 
made. In the year 1900 Professor de Vries published 
a brief account"^ of experiments which he has for several 
years been carrying on, giving results of the highest value. 

The description is very short, and there are several 
points as to which more precise information is necessary 
both as to details of procedure and as to statement of 
results. Nevertheless it is impossible to doubt that the 
work as a whole constitutes a marked step forward, and 
the full publication which is promised will be awaited with 
great interest. 

The work relates to the course of heredity in cases 
where definite varieties differing from each other in some 
one definite character are crossed together. The cases are 
all examples of discontinuous variation : that is to say, 
cases in which actual intermediates between the parent 
forms are not usually produced on crossing!. It is shown 
that the subsequent posterity obtained by self-fertilising 
these cross-breds or hybrids, or by breeding them with each 
other, break up into the original parent forms according to 
fixed numerical rule. 

* Comptes Rendus, March 26, 1900, and Ber. d. Deutsch. Bot. 
Ges. xviii. 1900, p. 83. 

t This conception of discontinuity is of course pre-Mendelian. 

8 The Problems 

Professor de Vries begins by reference to a remarkable 
memoir by Gregor Mendel"^, giving the results of his 
experiments in crossing varieties of Pisum sativum. These 
experiments of Mendel's were carried out on a large scale, 
his account of them is excellent and complete, and the 
principles which he was able to deduce from them will 
certainly play a conspicuous part in all future discussions 
of evolutionary problems. It is not a little remarkable 
that Mendel's work should have escaped notice, and been 
so long forgotten. 

For the purposes of his experiments Mendel selected 
seven pairs of characters as follows : — 

1 . Shape of ripe seed, whether round ; or angular and 

2. Colour of " endosperm " (cotyledons), whether some 
shade of yellow ; or a more or less intense green. 

3. Colour of the seed-skin, whether various shades of 
grey and grey-brown ; or white. 

4. Shape of seed-pod, whether simply inflated ; or 
deeply constricted between the seeds. 

5. Colour of unripe pod, whether a shade of green ; or 
bright yellow. 

6. Nature of inflorescence, whether the flowers are 
arranged along the axis of the plant ; or are terminal and 
form a kind of umbel. 

7. Length of stem, whether about 6 or 7 ft. long, or 
about f to Ij ft. 

Large numbers of crosses were made between Peas dif- 
fering in respect of one of each of these pairs of characters. 

* ' Versuche iib. Pflanzenhybriden ' in the Verli. d. Naturf. Ver. 
Brilnn, iv. 1865. 

of Heredity 9 

It was found that in each case the offspring of the cross 
exhibited the character of one of the parents in ahnost 
undiminished intensity, and intermediates which could not 
be at once referred to one or other of the parental forms 
were not found. 

In the case of each pair of characters there is thus 
one which in the first cross prevails to the exclusion of the 
other. This prevailing character Mendel calls the dominant 
character, the other being the recessim character*. 

That the existence of such "dominant" and "recessive" 
characters is a frequent phenomenon in cross-breeding, is 
well known to all who have attended to these subjects. 

By letting the cross-breds fertilise themselves Mendel 
next raised another generation. In this generation were 
individuals which showed the dominant character, but also 
individuals which presented the recessive character. Such 
a fact also was known in a good many instances. But 
Mendel discovered that in this generation the numerical 
proportion of dominants to recessives is on an average of 
cases approximately constant, being in fact as three to one. 
With very considerable regularity these numbers were 
approached in the case of each of his pairs of characters. 

There are thus in the first generation raised from the 
cross-breds 75 pei^ cent, dominants and 25 per cent, 

These plants were again self-fertilised, and the offspring 
of each plant separately sown. It next appeared that the 
offspring of the recessives remained pure recessive, and 
in subsequent generations never produced the dominant 

But when the seeds obtained by self-fertilising the 

* Note that by these novel terms the complications involved by 
use of the expression " prepotent " are avoided. 

10 The Problems 

dominants were examined and sown it was found that 
the dominants were not all alike, but consisted of two 
classes, (1) those which gave rise to pure dominants, and 
(2) others which gave a mixed offspring, composed partly 
of recessives, partly of dominants. Here also it was found 
that the average numerical proportions were constant, those 
with pure dominant offspring being to those with mixed 
offspring as one to two. Hence it is seen that the 75 per 
cent, dominants are not really of similar constitution, but 
consist of twenty-five which are pure dominants and fifty 
which are really cross-breds, though, like the cross- breds 
raised by crossing the two original varieties, they only 
exhibit the dominant character. 

To resume, then, it was found that by self-fertilising 
the original cross-breds the same proportion was always 
approached, namely — 

25 dominants, 50 cross-breds, 25 recessives, 

or \D : 2DR : \R. 

Like the pure recessives, the pure dominants are 
thenceforth pure, and only give rise to dominants in all 
succeeding generations studied. 

On the contrary the fifty cross-breds, as stated above, 
have mixed offspring. But these offspring, again, in their 
numerical proportions, follow the same law, namely, that 
there are three dominants to one recessive. The recessives 
are pure like those of the last generation, but the dominants 
can, by further self-fertilisation, and examination or culti- 
vation of the seeds produced, be again shown to be made 
up of pure dominants and cross-breds in the same proportion 
of one dominant to two cross-breds. 

The process of breaking up into the parent forms is 
thus continued in each successive generation, the same 

of Heredity 11 

numerical law being followed so far as has yet been 
Mendel made further experiments with Pisum sativum, 
crossing pairs of varieties which differed from each other 
in two characters, and the results, though necessarily much 
more complex, showed that the law exhibited in the simpler 
case of pairs differing in respect of one character operated 
here also. 

In the case of the union of varieties AB and ah 
differing in two distinct pairs of characters, A and a, 
B and h, of which A and B are dominant, a and h 
recessive, Mendel found that in the first cross-bred gene- 
ration there was only one class of offspring, really AaBh. 

But by reason of the dominance of one character of 
each pair these first crosses were hardly if at all distin- 
guishable from AB. 

By letting these AaBb'^ fertilise themselves, only four 
classes of offspring seemed to be produced, namely, 

AB showing both dominant characters. 
Ah „ dominant J. and recessive h. 
aB „ recessive a and dominant B. 
ah „ both recessive characters a and h. 

The numerical ratio in which these classes appeared 
were also regular and approached the ratio 

^AB '.^Ah: MB : lah. 

But on cultivating these plants and allowing them to 
fertilise themselves it was found that the members of the 


1 ah class produce only a6's. 

1 aB class may produce either all a^'s, 
,2 or both aB'^ and a6's. 

12 The Problems 


1 Ah class may produce either all J.6's, 
^2 or both Ah'^ and ah'^. 

1 AB class may produce either all J.^'s, 

2 or both ^5's and AU^, 
H 2 or both AB'^ and a^'s, 

or all four possible classes again, namely, 
AB'&, J.^'s, aB^y and a6's, 

and the average number of members of each class will 
approach the ratio 1 : 3 : 3 : 9 as indicated above. 

The details of these experiments and of others like 
them made with three pairs of differentiating characters are 
all set out in Mendel's memoir. 

Professor de Vries has worked at the same problem in 
some dozen species belonging to several genera, using pairs 
of varieties characterised by a great number of characters : 
for instance, colour of flowers, stems, or fruits, hairiness, 
length of style, and so forth. He states that in all these 
cases Mendel's principles are followed. 

The numbers with which Mendel worked, though large, 
were not large enough to give really smooth results * ; but 
with a few rather marked exceptions the observations are 
remarkably consistent, and the approximation to the num- 
bers demanded by the law is greatest in those cases where 
the largest numbers were used. When we consider, besides, 
that Tschermak and Correns announce definite confirmation 
in the case of Pisum, and de Vries adds the evidence of his 
long series of observations on other species and orders, 
there can be no doubt that Mendel's law is a substantial 

* Professor Weldon (p. 232) takes great exception to this state- 
ment, which he considerately attributes to " some writers." After 
examining the conclusions he obtained by algebraical study of Mendel's 
figures I am disposed to think my statement not very far out. 

of Heredity 13 

reality ; thougli whether some of the cases that depart 
most widely from it can be brought within the terms of 
the same principle or not, can only be decided by further 

One may naturally ask, How can these results be 
brought into harmony with the facts of hybridisation 
hitherto known ; and, if all this is true, how is it that 
others who have carefully studied the phenomena of hy- 
bridisation have not long ago perceived this law? The 
answer to this question is given by Mendel at some length, 
and it is, I think, satisfactory. He admits from the first 
that there are undoubtedly cases of hybrids and cross-breds 
which maintain themselves pure and do not break up. 
Such examples are plainly outside the scope of his law. 
Next he points out, what to anyone who has rightly 
comprehended the nature of discontinuity in variation is 
well known, that the variations in each character must be 
separately regarded. In most experiments in crossing, 
forms are taken which differ from each other in a multi- 
tude of characters — some continuous, others discontinuous, 
some capable of blending with their contraries, while others 
are not. The observer on attempting to perceive any 
regularity is confused by the complications thus intro- 
duced. Mendel's law, as he fairly says, could only appear 
in such cases by the use of overwhelming numbers, which 
are beyond the possibilities of practical experiment. Lastly, 
no previous observer had applied a strict statistical method. 

Both these answers should be acceptable to those who 
have studied the facts of variation and have appreciated 
the nature of Species in the light of those facts. That 
different species should follow different laws, and that the 
same law should not apply to all characters alike, is exactly 
what we have every right to expect. It will also be 

14 The Problems 

remembered that the principle is only explicitly declared 
to apply to discontinuous characters*. As stated also 
it can only be true where reciprocal crossings lead to the 
same result. Moreover, it can only be tested when there 
is no sensible diminution in fertility on crossing. 

Upon the appearance of de Vries' paper announcing the 
"rediscovery" and confirmation of Mendel's law and its 
extension to a great number of cases two other observers 
came forward almost simultaneously and independently 
described series of experiments fully confirming Mendel's 
work. Of these papers the first is that of Correns, who 
repeated Mendel's original experiment with Peas having 
seeds of different colours. The second is a long and very 
valuable memoir of Tschermak, which gives an account of 
elaborate researches into the results of crossing a number 
of varieties of Pisum sativum. These experiments were in 
many cases carried out on a large scale, and prove the 
main fact enuntiated by Mendel beyond any possibility of 
contradiction. The more exhaustive of these researches 
are those of Tschermak on Peas and Correns on several 
varieties of Maize. Both these elaborate investigations 
have abundantly proved the general applicability of Mendel's 
law to the character of the plants studied, though both 
indicate some few exceptions. The details of de Vries' 
experiments are promised in the second volume of his most 
valuable Mutationstheorie. Correns in regard to Maize 
and Tschermak in the case of P. sativum have obtained 
further proof that Mendel's law holds as well in the case of 
varieties differing from each other in two pairs of characters, 
one of each pair being dominant, though of course a more 
complicated expression is needed in such cases f. 

* See later. 

t Tschermak's investigations were besides directed to a re-exami- 

of Heredity 15 

That we are in the presence of a new principle of the 
highest importance is manifest. To what further con- 
clusions it may lead us cannot yet be foretold. But both 
Mendel and the authors who have followed him lay stress 
on one conclusion, which will at once suggest itself to 
anyone who reflects on the facts. For it will be seen that 
the results are such as we might expect if it be imagined 
that the cross-bred plant produced pollen grains and egg- 
cells, each of which bears only one of the alternative varietal 
characters and not both. If this were so, and if on an 
average the same number of pollen grains and egg-cells 
transmit each of the two characters, it is clear that on a 
random assortment of pollen grains and egg-cells Mendel's 
law would be obeyed. For 25 per cent, of "dominant" 
pollen grains would unite with 25 per cent, "dominant" 
egg-cells ; 25 per cent. " recessive " pollen grains would 
similarly unite with 25 per cent. " recessive " egg-cells ; 
while the remaining 50 per cent, of each kind would unite 
together. It is this consideration which leads both Mendel 
and those who have followed him to assert that these facts 
of crossing prove that each egg-cell and each pollen grain 
is pure in respect of each character to which the law 
applies. It is highly desirable that varieties differing in 
the form of their pollen should be made the subject of 
these experiments, for it is quite possible that in such a 
case strong confirmation of this deduction might be ob- 
tained. [Preliminary trials made with reference to this 
point have so far given negative results. Eemembering 
that a pollen grain is not a germ-cell, but only a bearer of 

nation of the question of the absence of beneficial results on cross- 
fertilising P. sativum, a subject already much investigated by Darwin, 
and upon this matter also important further evidence is given in 
great detail. 

16 The Problems 

a germ-cell, the hope of seeing pollen grains differentiated 
according to the characters they bear is probably remote. 
Better hopes may perhaps be entertained in regard to 
spermatozoa, or possibly female cells.] 

As an objection to the deduction of purity of germ-cells, 
however, it is to be noted that though true intermediates 
did not generally occur, yet the intensity in which the 
characters appeared did vary in degree, and it is not easy 
to see how the hypothesis of perfect purity in the repro- 
ductive cells can be supported in such cases. Be this, 
however, as it may, there is no doubt we are beginning to 
get new lights of a most valuable kind on the nature of 
heredity and the laws which it obeys. It is to be hoped 
that these indications will be at once followed up by 
independent workers. Enough has been said to show how 
necessary it is that the subjects of experiment should be 
chosen in such a way as to bring the laws of heredity to a 
real test. For this purpose the first essential is that the 
differentiating characters should be few, and that all avoid- 
able complications should be got rid of. Each experiment 
should be reduced to its simplest possible limits. The 
results obtained by Galton, and also the new ones especially 
described in this paper, have each been reached by restricting 
the range of observation to one character or group of char- 
acters, and it is certain that by similar treatment our 
knowledge of heredity may be rapidly extended. 

To the above popular presentation of the essential facts, 
made for an audience not strictly scientific, some addition, 
however brief, is called for. First, in regard to the law of 
Ancestry, spoken of on p. 5. Those who are acquainted with 
Pearson's Grammar of Science, 2nd ed. pubHshed early in 

of Heredity 17 

1900, the same author's paper in Proc. R. S. vol. 66, 1900, 
p. 140, or the extensive memoir (pubd. Oct. 1900), on the 
inheritance of coat-colour in horses and eye-colour in man 
(Fkil. Trans. 195, a, 1900, p. 79), will not need to be told 
that the few words I have given above constitute a most 
imperfect diagram of the operations of that law as now de- 
veloped. Until the appearance of these treatises it was, 
I believe, generally considered that the law of Ancestral 
Heredity was to be taken as applying to phenomena like 
these (coat-colour, eye-colour, &c.) where the inheritance 
is generally alternative, as well as to the phenomena 
of hlended inheritance. 

Pearson, in the writings referred to, besides withdrawing 
other large categories of phenomena from the scope of its 
operations, points out that the law of Ancestral Heredity 
does not satisfactorily express the cases of alternative 
inheritance. He urges, and with reason, that these classes 
of phenomena should be separately dealt with. 

The whole issue as regards the various possibilities of 
heredity now recognized will be made clearer by a very brief 
exposition of the several conceptions involved. 

If an organism producing germ-cells of a given constitu- 
tion, uniform in respect of the characters they bear, breeds 
with another organism* bearing precisely similar germ- 
cells, the offspring resulting will, if the conditions are 
identical, be uniform. 

In practice such a phenomenon is seen in jl?^«rg-breeding. 
It is true that we know no case in nature where all the 
germ-cells are thus identical, and where no variation takes 
place beyond what we can attribute to conditions, but we 

* For simplicity the case of self-fertilisation is omitted from this 

B. 2 

18 The Problems 

know many cases where such a result is approached, and 
very many where all the essential features which we regard 
as constituting the characters of the breed are reproduced 
with approximate certainty in every member of the pure- 
bred race, which thus closely approach to uniformity. 

But if two germ-cells of dissimilar constitution unite 
in fertilisation, what offspring are we to expect*? First 
let us premise that the answer to this question is known 
experimentally to differ for many organisms and for many 
classes of characters, and may almost certainly be in part 
determined by external circumstances. But omitting the 
last qualification, certain principles are now clearly detected, 
though what principle will apply in any given case can only 
be determined by direct experiment made with that case. 

This is the phenomenon of ^ ross-breeding. As generally 
used, this term means the union of members of dissimilar 
varieties, or sj^cies : though when dissimilar gametes t pro- 
duced by two individuals of the same variety unite in 
fertilisation, we have essentially cross-breeding in respect 
of the character or characters in which those gametes differ. 
We will suppose, as before, that these two gametes bearing 
properties unlike in respect of a given character, are borne 
hy different individuals. 

In the simplest case, suppose a gamete from an in- 
dividual presenting any character in intensity A unite in 
fertilisation with another from an individual presenting 
the same character in intensity a. For brevity's sake we 

* In all the cases discussed it is assumed that the gametes are 
similar except in regard to the ' ' heritage " they bear, and that no 
original variation is taking place. The case of mosaics is also left 
wholly out of account (see later). 

t The term "gamete" is now generally used as the equivalent of 
"germ-cell," whether male or female, and the term "zygote" is here 
used for brevity to denote the organism resulting from fertilisation. 

of Heredity 19 

may call the parent individuals A and a, and the resulting 
zygote Aa. "What will the structure of Aa be in regard to 
the character we are considering ? 

Up to Mendel no one proposed to answer this question 
in any other way than by reference to the intensity of the 
character in the progenitors, and 'primarily in the parents, 
A and a, in whose bodies the gametes had been developed. 
It was well known that such a reference gave a very poor 
indication of what Aa would be. Both A and a may come 
from a population consisting of individuals manifesting the 
same character in various intensities. In the pedigree of 
either A ot a these various intensities may have occurred 
few or many times. Common experience leads us to expect 
the probability in regard to Aa tQ> be influenced by this 
history. The next step is that which Gait on took. He 
extended the reference beyond the immediate parents of 
Aa, to its grandparents, great-grandparents, and so on, and 
in the cases he studied he found that from a knowledge of 
the intensity in which the given character was manifested 
in each progenitor, even for some few generations back, a 
fairly accurate prediction could be made, not as to the 
character of any individual Aa, but as to the average 
character of Aa'^ of similar parentage, in general. 

But suppose that instead of individuals presenting one 
character in differing intensities, two individuals breed 
together distinguished by characters which we know to be 
mutually exclusive, such as A and B. Here again we may 
speak of the individuals producing the gametes as A and 
B, and the resulting zygote as AB. What will ^^ be 
like ? The population here again may consist of many like 
A and like B. These two forms may have been breeding 
together indiscriminately, and there may have been many 
or few of either type in the pedigree of either A or B. 


20 The Problems 

Here again Galton applied his method with remarkable 
success. Referring to the progenitors of A and B^ deter- 
mining how many of each type there were in the direct 
pedigree of A and of B, he arrived at the same formula as 
before, with the simple difference that instead of expressing 
the probable average intensity of one character in several 
individuals, the prediction is given in terms of the probable 
number of 4's and 5's that would result on an average 
when particular J.'s and J5's of known pedigree breed 

The law as Galton gives it is as follows : — 
"It is that the two parents contribute between them 
on the average one-half, or (0'5) of the total heritage of 
the offspring; the four grandparents, one-quarter, or (0*5)^; 
the eight great-grandparents, one-eighth, or (0*5)^, and so 
on. Then the sum of the ancestral contributions is ex- 
pressed by the series 

{(0-5) + (0-5)^ + (0-5)^&c.}, 

which, being equal to 1, accounts for the whole heritage." 

In the former case where A and a are characters which 
can be denoted by reference to a common scale, the law 
assumes of course that the inheritance will be, to use 
Galton' s term, blended ^ namely that the zygote resulting 
from the union of A with a wdll on the average be more 
like a than if A had been united with A ; and conversely 
that dji. Aa zygote will on the average he more like A than 
an aa zygote would he. 

But in the case of J.'s and jS's, which are assumed to 
be mutually exclusive characters, we cannot speak of 
blending, but rather, to use Galton's term, of alternative 

Pearson, finding that the law whether formulated thus, 

of Heredity 21 

or in the modified form in which he restated it*, did not 
express the phenomena of alternative inheritance known 
to him with sufficient accuracy to justify its strict appli- 
cation to them, and also on general grounds, proposed that 
the phenomena of blended and alternative inheritance 
should be treated apart — a suggestion t the "vvisdom of 
which can scarcely be questioned. 

Now the law thus imperfectly set forth and every 
modification of it is incomplete in one respect. It deals 
only Avith the characters of the resulting zygotes and 
predicates nothing in regard to the gametes which go to 
form them. A good prediction may be made as to any 
given group of zygotes, but the various possible constitu- 
tions of the gametes are not explicitly treated. 

Nevertheless a definite assumption is implicitly made 
regarding the gametes. It is not in question that differences 
between these gametes may occur in respect of the heritage 
they bear ; yet it is assumed that these differences will be 
distributed among the gametes of any individual zygote in 
such a way that each gamete remains capable, on fertilisa- 
tion, of transmitting all the characters (both of the parent- 
zygote and of its progenitors) to the zygote which it then 
contributes to form (and to the posterity of that zygote) in 
the intensity indicated by the law. Hence the gametes of 
any individual are taken as collectively a fair sample of all 
the racial characters in their appropriate intensities, and this 
theory demands that there shall have been no qualitative 
redistribution of characters among the gametes of any 
zygote in such a way that some gametes shall be finally 
excluded from partaking of and transmitting any specific 

* In Pearson's modification the parents contribute 0*3, the grand- 
parents O'lo, the great-grandparents -075. 
t See the works referred to above. 

22 The Problems 

part of the heritage. The theory further demands — and 
by the analogy of what we know otherwise not only of 
animals and plants, but of physical or chemical laws, 
perhaps this is the most serious assumption of all — that 
the structure of the gametes shall admit of their being 
capable of transmitting any character in any intensity 
varying from zero to totality with equal ease ; and that 
gametes of each intensity are all equally likely to occur, 
given a pedigree of appropriate arithmetical composition. 

Such an assumption appears so improbable that even 
in cases where the facts seem as yet to point to this 
conclusion with exceptional clearness, as in the case of 
human stature, I cannot but feel there is still room for 
reserve of judgment. 

However this may be, the Law of Ancestral Heredity, 
and all modifications of it yet proposed, falls short in the 
respect specified above, that it does not directly attempt 
to give any account of the distribution of the heritage among 
the gametes of any one individual. 

Mendel's conception differs fundamentally from that 
involved in the Law of Ancestral Heredity. The relation 
of his hypothesis to the foregoing may be most easily 
shown if we consider it first in application to the pheno- 
mena resulting from the cross-breeding of two pure 

Let us again consider the case of two varieties each dis- 
playing the same character, but in the respective intensities 
A and a. Each gamete of the A variety bears A, and 
each gamete of the a variety bears a. When they unite in 
fertilisation they form the zygote Aa. What will be its 
characters ? The Mendelian teaching would reply that 
this can only be known by direct experiment with the two 
forms A and a, and that the characters A and a perceived 

of Heredity 23 

in those two forms or varieties need not give any indication 
as to the character of the zygote Aa. It may display the 
character J., or a, or a character half way between the two, 
or a character beyond A or below a. The character of J. a 
is not regarded as a heritage transmitted to it by A and by 
«, but as a character special and peculiar to Aa^ just as 
NaCl is not a body half way between sodium and chlorine, 
or such that its properties can be predicted from or easily 
stated in terms of theirs. 

If a concrete case may help, a tall pea A crossed with 
a dwarf a often produces, not a plant having the height of 
either A or a, but something taller than the pure tall 
variety A. 

But if the case obeys the Mendelian principles — as does 
that here quoted — then it can be declared first that the 
gametes of Aa will not be bearers of the character proper to 
Aa ; but, generally speaking, each gamete will either bear 
the pure A character or the pure a character. There will 
in fact be a redistribution of the characters brought in by 
the gametes which united to form the zygote Aa, such that 
each gamete oi Aa is pure, as the parental gametes were. 
Secondly this redistribution will occur in such a way that, 
of the gametes produced by such Jla's, on an average 
there will be equal numbers of A gametes and of a 

Consequently if J.a's breed together, the new A gametes 
may meet each other in fertilisation, forming a zygote A A, 
namely, the pure A variety again ; similarly two a gametes 
may meet and form aa, or the pure a variety again. Bat if 
an A gamete meets an a it will once more form Aa, with 
its special character. This Aa is the hybrid, or "mule" 
form, or as I have elsewhere called it, the heterozygote, as 
distinguished from A A ov aa the komozygotes. 

24 The Problems 

Similarly if the two gametes of two varieties distin- 
guished by characters, A and B, which cannot be described 
in terms of any common scale (such as for example the 
" rose " and " single " combs of fowls) unite in fertilisation, 
again the character of the mule form cannot be predicted. 
Before the experiment is made the "mule" may present ani/ 
form. Its character or properties can as yet be no more 
predicted than could those of the compounds of unknown 
elements before the discovery of the periodic law. 

But again — if the case be Mendelian — the gametes borne 
by AB will be either A's or ^'s*, and the cross-bred 
^^'s breeding together will form AA's, AB's and ^^'s. 
Moreover, if as in the normal Mendelian case, AB's bear on 
an average equal numbers of A gametes and B gametes, the 
numerical ratio of these resulting zygotes to each other will be 

lAA :2AB -.IBB. 

We have seen that Mendel makes no prediction as to 
the outward and visible characters of AB, but only as 
to the essential constitution and statistical condition of its 
gametes in regard to the characters A and B. Nevertheless 
in a large number of cases the character of AB is known 
to fall into one of three categories (omitting mosaics). 

(1) The cross-bred may almost always resemble one 
of its pure parents so closely as to be practically 
indistinguishable from that pure form, as in the 
case of the yellow cotyledon-colour of certain varieties 
of peas when crossed with green-cotyledoned varieties ; 
in which case the parental character, yellow, thus 

* This conception was clearly formed by Naudin simultaneously 
with Mendel, but it was not worked out by him and remained a mere 
suggestion. In one place also Focke came very near to the same idea 
(see Bibliography). 

of Heredity 25 

manifested by the cross-bred is called "dominant" 
and the parental character, green, not manifested, is 
called recessive. 

(2) The cross-bred may present some condition 
intermediate between the two parental forms, in 
which case we may still retain the term "blend" 
as applied to the zygote. 

Such an "intermediate" may be the apparent mean 
between the two parental forms or be nearer to one 
or other in any degree. Such a case is that of a 
cross between a rich crimson Magenta Chinese Prim- 
rose and a clear White, giving a flower of a colour 
appropriately described as a "washy" magenta. 

(3) The cross-bred may present some form quite 
different from that of either pure parent. Though, 
as has been stated, nothing can be predicted of an un- 
known case, we already know a considerable number 
of examples of this nature in which the mule-form 
approaches sometimes with great accuracy to that of 
a putative ancestor, near or remote. It is scarcely 
possible to doubt that several — though perhaps not 
all — of Darwin's "reversions on crossing" were of 
this nature. 

Such a case is that of the "wild grey mouse " produced 
by the union of an albino tame mouse and a piebald 
Japanese mouse*. These "reversionary" mice bred 
together produce the parental tame types, some other 
types, and "reversionary" mice again. 

From what has been said it will now be clear that the 
appHcabihty of the Mendelian hypothesis has, intrinsically, 

* See von Guaita, Ber. natnrf. Gen. Freilmrg x. 1898 and xi. 1899, 
quoted by Professor Weldon (see later). 

26 The Problems 

nothing whatever to do with the question of the inheritance 
being blended or alternative. In fact, as soon as the relation 
of zygote characters to gamete characters is appreciated, it is 
difficult to see any reason for supposing that the manifes- 
tation of characters seen in the zygotes should give any 
indication as to their mode of allotment among the gametes. 
On a previous occasion I pointed out that the terms 
"Heredity" and "Inheritance" are founded on a mis- 
application of metaphor, and in the light of our present 
knowledge it is becoming clearer that the ideas of "trans- 
mission " of a character by parent to offspring, or of there 
being any "contribution" made by an ancestor to its pos- 
terity, must only be admitted under the strictest reserve, 
and merely as descriptive terms. 

We are now presented with some entirely new con- 
ceptions : — 

(1) The purity of the gametes in regard to certain 

(2) The distinction of all zygotes according as they are or 
are not formed by the union of like or unlike gametes. 
In the former case, apart from Variation, they breed 
true when mated with their like ; in the latter case 
their offspring, collectively, will be heterogeneous. 

(3) If the zygote be formed by the union of dissimilar 
gametes, we may meet the phenomenon of (a) domi- 
nant and recessive characters ; (b) a blend form ; 
(c) a form distinct from either parent, often 

* This fact sufficiently indicates the difficulties involved in a 
superficial treatment of the phenomenon of reversion. To call such 
reversions as those named above " returns to ancestral type " would 
be, if more than a descriptive phrase were intended, quite misleading. 

of Heredity 27 

But there are additional and even more significant de- 
ductions from the facts. We have seen that the gametes are 
differentiated in respect of pure characters. Of these pure 
characters there may conceivably be any number associated 
together in one organism. In the pea Mendel detected at 
least seven — not all seen by him combined in the same 
plant, but there is every likelihood that they are all capable 
of being thus combined. 

Each such character, which is capable of being dissociated 
or replaced by its contrary, must henceforth be conceived 
of as a distinct unit-character ; and as we know that the 
several unit-characters are of such a nature that any one 
of them is capable of independently displacing or being dis- 
placed by one or more alternative characters taken singly, 
we may recognize this fact by naming such unit-characters 
allelomorphs. So far, we know very little of any allelomorphs 
existing otherwise than as pairs of contraries, but this is 
probably merely due to experimental limitations and the 
rudimentary state of our knowledge. 

In one case (combs of fowls) we know three characters, 
pea comb, rose comb and single comb ; of which pea and 
single, or rose and single, behave towards each other as a 
pair of allelomorphs, but of the behaviour of pea and rose 
towards each other we know as yet nothing. 

We have no reason as yet for affirming that any 
phenomenon properly described as displacement of one 
allelomorph by another occurs, though the metaphor may 
be a useful one. In all cases where dominance has been 
perceived, we can affirm that the members of the allelo- 
morphic pair stand to each other in a relation the nature 

It is not the ancestral type that has come back, but something else 
has come in its guise, as the offspring presently prove. For the first 
time we thus begin to get a rationale of " reversion." 

28 The Problems 

of whicli we are as yet wholly unable to apprehend or 

To the new conceptions already enumerated we may 
therefore add 

(4) Unit-characters of which some, when once arisen by 
Variation, are alternative to each other in the consti- 
tution of the gametes, according to a definite system. 

From the relations subsisting between these characters, 
it follows that as each zygotic union of allelomorphs is re- 
solved on the formation of the gametes, no zygote can give 
rise to gametes collectively representing more than two cha- 
racters allelomorphic to each other, apart from new variation. 

From the fact of the existence of the interchangeable 
characters we must, for purposes of treatment, and to com- 
plete the possibilities, necessarily form the conception of an 
irresoluble base, though whether such a conception has any 
objective reality we have no means as yet of determining. 

We have now seen that when the varieties A and B 
are crossed together, the heterozygote, AB, produces 
gametes bearing the pure A character and the pure B 
character. In such a case we speak of such characters as 
simple allelomorphs. In many cases however a more 
complex phenomenon happens. The character brought in 
on fertilisation by one or other parent may be of such a 
nature that when the zygote, AB, forms its gametes, these 
are not individually bearers merely of A and B, but of a 
number of characters themsehes again integral, which in, 
say A, behaved as one character so long as its gametes 
united in fertilisation with others Hke themselves, but on 
cross-fertilisation are resolved and redistributed among the 
gametes produced by the cross-bred zygote. 

In such a case we call the character A a compound 

of Heredity 29 

allelomorph, and we can speak of the integral characters 
which constitute it as hypallelomorpJis. We ought to write 
the heterozygote {AA'A"...) B and the gametes produced 
by it may be of the form A, A', A", A"\...B. Or the 
resolution may be incomplete in various degrees, as we 
already suspect from certain instances ; in which case we 
may have gametes A, A'A'\ A!" A"" , A'A'A\...B, and 
so on. Each of these may meet a similar or a dissimilar 
gamete in fertilisation, forming either a homozygote, or a 
heterozygote with its distinct properties. 

In the case of compound allelomorphs we know as yet 
nothing of the statistical relations of the several gametes. 

Thus we have the conception 

(5) of a Compound character^ borne by one gamete, 
transmitted entire as a single character so long as 
fertilisation only occurs between like gametes, or is, 
in other words, ''symmetrical," but if fertilisation 
take place with a dissimilar gamete (or possibly by 
other causes), resolved into integral constituent- 
characters, each separately transmissible. 

Next, as, by the union of the gametes bearing the 
various hypallelomorphs with other such gametes, or with 
gametes bearing simple allelomorphs, in fertilisation, a 
number of new zygotes will be formed, such as may not have 
been seen before in the breed : these will inevitably be 
spoken of as varieties ; and it is difficult not to extend the 
idea of variation to them. To distinguish these from other 
variations — which there must surely be — we may call them 

(6) Analytical variations in contradistinction to 

(7) Synthetical variations, occurring not by the 
separation of pre-existing constituent-characters but 
by the addition of new characters. 

30 The Problems 

Lastly, it is impossible to be presented with the fact 
that in Mendelian cases the cross-bred produces on an 
average equal numbers of gametes of each kind, that is to 
say, a symmetrical result, without suspecting that this fact 
must correspond with some symmetrical figure of distribu- 
tion of those gametes in the cell-divisions by which they are 

At the present time these are the main conceptions — 
though by no means all — arising directly from Mendel's 
work. The first six are all more or less clearly embodied 
by him, though not in every case developed in accordance 
with modern knowledge. The seventh is not a Mendelian 
conception, but the facts before us justify its inclusion in 
the above list though for the present it is little more than 
a mere surmise. 

In Mendelian cases it will now be perceived that all 
the zygotes composing the population consist of a limited 
number of possible types, each of definite constitution, 
bearing gametes also of a limited and definite number of 
types, and definite constitution in respect of pre-existing 
characters. It is now evident that in such cases each 
several progenitor need not be brought to account in 
reckoning the probable characters of each descendant ; 
for the gametes of cross-breds are differentiated at each 
successive generation, some parental (Mendelian) characters 
being left out in the composition of each gamete produced 
by a zygote arising by the union of bearers of opposite 

When from these considerations we return to the 
phenomena comprised in the Law of Ancestral Heredity, 
what certainty have we that the same conceptions are not 
applicable there also ? 

of Heredity 31 

It has now been shown that the question whether in the 
cross-bred zygotes in general the characters blend or are 
mutually exclusive is an entirely subordinate one, and 
distinctions with regard to the essential nature of heredity 
based on these circumstances become irrelevant. 

In the case of a population presenting continuous 
variation in regard to say, stature, it is easy to see how 
purity of the gametes in respect of any intensities of 
that character might not in ordinary circumstances be 
capable of detection. There are doubtless more than 
two pure gametic forms of this character, but there may 
quite conceivably be six or eight. When it is remem- 
bered that each heterozygous combination of any two 
may have its own appropriate stature, and that such a 
character is distinctly dependent on external conditions, 
the mere fact that the observed curves of stature give 
"chance distributions" is not surprising and may still be 
compatible with purity of gametes in respect of certain 
pure types. In peas {P. sativum), for example, from 
Mendel's work we know that the tall forms and the ex- 
treme dwarf forms exhibit gametic purity. I have seen 
at Messrs Sutton's strong evidence of the same nature 
in the case of the tall Sweet Pea {Lathyrus odoratus) 
and the dwarf or procumbent "Cupid" form. 

But in the case of the Sweet Pea we know at least one 
pure form of definitely intermediate height, and in the 
case of P. sativum there are many. When the extreme 
types breed together it will be remembered the heterozygote 
commonly exceeds the taller in height. In the next 
generation, since there is, in the case of extremes, so much 
margin between the types of the two pure forms, the return 
of the offspring to the three forms of which two are homo- 
zygous and one heterozygous is clearly perceptible. 

32 The Problems 

If however instead of pure extreme varieties we were to 
take a pair of varieties differing normally by only a foot or 
two, we might, owing to the masking effects of conditions, 
&c., have great difficulty in distinguishing the three forms 
in the second generation. There would besides be twice as 
many heterozygous individuals as homozygous individuals 
of each kind, giving a symmetrical distribution of heights, 
and who might not — in pre-Mendelian days — have accepted 
such evidence — made still less clear by influence of con- 
ditions — as proof of Continuous Variation both of zygotes 
and gametes ? 

Suppose, then, that instead of two pure types, we had 
six or eight breeding together, each pair forming their own 
heterozygote, there would be a very remote chance of such 
purity or fixity of type whether of gamete or zygote being 

Dominance, as we have seen, is merely a phenomenon 
incidental to specific cases, between which no other common 
property has yet been perceived. In the phenomena of 
blended inheritance we clearly have no dominance. In the 
cases of alternative inheritance studied by Galton and 
Pearson there is evidently no universal dominance. From 
the tables of Basset hound pedigrees there is clearly no 
definite dominance of either of the coat-colours. In the case 
of eye-colour the published tables do not, so far as I have 
discovered, furnish the material for a decision, though it is 
scarcely possible the phenomenon, even if only occasional, 
could have been overlooked. We must take it, then, there 
is no sensible dominance in these cases : but whether there 
is or is not sensible gametic purity is an altogether different 
question, which, so far as I can judge, is as yet untouched. 
It may perfectly well be that we shall be compelled to 
recognize that in many cases there is no such purity, and 

of Heredity 33 

that the characters may be carried by the gametes in any 
proportion from zero to totahty, just as some substances 
may be carried in a solution in any proportion from zero 
to saturation without discontinuous change of properties. 
That this will be found true in some cases is, on any 
hypothesis, certain ; but to prove the fact for any given 
case will be an exceedingly difficult operation, and I scarcely 
think it has been yet carried through in such a way as to 
leave no room for doubt. 

Conversely, the absolute and iinivei^sal purity of the 
gametes has certainly not yet been determined for any 
case ; not even in those cases where it looks most likely 
that such universal purity exists. Impairment of such 
purity we may conceive either to occur in the form of 
mosaic gametes, or of gametes with blended properties. 
On analogy and from direct evidence we have every right 
to believe that gametes of both these classes may occur in 
rare and exceptional cases, of as yet unexplored nature*, 
but such a phenomenon will not diminish the significance 
of observed purity. 

We have now seen the essential nature of the Mendelian 
principles and are able to appreciate the exact relation in 
which they stand to the gToup of cases included in the Law 
of Ancestral Heredity. In seeking any general indication 
as to the common properties of the phenomena which are 
already know^n to obey Mendelian principles we can as yet 
point to none, and whether some such common features 
exist or not is unknown. 

There is however one group of cases, definite though 
as yet not numerous, where we know that the Mendelian 

* It will be understood from what follows, that the existence of 
mosaic zygotes is no proof that either component gamete was mosaic. 

B. 3 

34 The Problems 

principles do not apply. These are the phenomena upon 
which Mendel touches in his brief paper on Hieracium. 
As he there states, the hybrids, if they are fertile at all, 
produce offspring like themselves, not like their parents. 
In further illustration of this phenomenon he cites Wichura's 
Salix hybrids. Perhaps some dozen other such illustrations 
could be given which rest on good evidence. To these 
cases the Mendelian principle will in nowise apply, nor is it 
easy to conceive any modification of the law of ancestral 
heredity which can express them. There the matter at 
present rests. Among these cases, however, we perceive 
several more or less common features. They are often, 
though not always, hybrids between forms differing in 
many characters. The first cross frequently is not the 
exact intermediate between the two parental types, but 
may as in the few Hieracium cases be irregular in this 
respect. There is often some degree of sterility. In the 
absence of fuller and statistical knowledge of such cases 
further discussion is impossible. 

Another class of cases, untouched by any hypothesis of 
heredity yet propounded, is that of the false hybrids of 
Millardet, where we have fertilisation without transmission 
of one or several parental characters. In these not only 
does the first cross show, in some respect, the character or 
characters of one parent only, but in its posterity no re- 
appearance of the lost character or characters is observed. 
The nature of such cases is still quite obscure, but we have 
to suppose that the allelomorph of one gamete only developes 
after fertilisation to the exclusion of the corresponding alle- 
lomorph of the other gamete, much — if the crudity of the 
comparison may be pardoned — as occurs on the female side 
in parthenogenesis without fertilisation at all. 

of Heredity 35 

To these as yet altogether unconformable cases we can 
scarcely doubt that further experiment will add many more. 
Indeed we already have tolerably clear evidence that many 
phenomena of inheritance are of a much higher order of 
complexity. When the paper on Pisum was written 
Mendel apparently inclined to the view that with modi- 
fications his law might be found to include all the phenomena 
of hybridisation, but in the brief subsequent paper on 
Hieracium he clearly recognized the existence of cases of 
a different nature. Those who read that contribution will 
be interested to see that he lays down a principle which 
may be extended from hybridisation to heredity in general, 
that the laws of each new case must be determined by 
separate experiment. 

As regards the Mendelian principles, which it is the 
chief aim of this introduction to present clearly before the 
reader, a professed student of variation will easily be able 
to fill in the outline now indicated, and to illustrate the 
various conceptions from phenomena already familiar. To 
do this is beyond the scope of this short sketch. But 
enough perhaps has now been said to show that by the 
application of those principles we are enabled to reach and 
deal in a comprehensive manner with phenomena of a 
fundamental nature, lying at the very root of all con- 
ceptions not merely of the physiology of reproduction 
and heredity, but even of the essential nature of living 
organisms ; and I think that I used no extravagant words 
when, in introducing Mendel's work to the notice of readers 
of the Royal Horticultural Society's Journal, I ventured to 
declare that his experiments are worthy to rank with those 
which laid the foundation of the Atomic laws of Chemistry. 


36 Brief Historical Notice 

As some biographical particulars of this remarkable 
investigator will be welcome, I give the following brief 
notice, first published by Dr Correns on the authority 
of Dr von Schanz : Gregor Johann Mendel was born on 
July 22, 1822, at Heinzendorf bei Odrau, in Austrian 
Silesia. He was the son of well-to-do peasants. In 1843 
he entered as a novice the "Koniginkloster," an Augustinian 
foundation in Altbriinn. In 1847 he was ordained priest. 
From 1851 to 1853 he studied physics and natural science 
at Vienna. Thence he returned to his cloister and became 
a teacher in the Realschule at Brlinn. Subsequently he 
was made Abbot, and died January 6, 1884, The experi- 
ments described in his papers were carried out in the 
garden of his Cloister. Besides the two papers on hybridi- 
sation, dealing respectively with Pisum and Hieracium, 
Mendel contributed two brief notes to the Verh. Zool. hot. 
Verein^ Wien, on Scopolia margarltalis (1853, iii., p. 116) 
and on Bruchus pisi {ibid. 1854, iv., p. 27). In these 
papers he speaks of himself as a pupil of KoUar. 

Mendel published in the Briinn journal statistical 
observations of a meteorological character, but, so far 
as I am aware, no others relating to natural history. 
Dr Correns tells me that in the latter part of his life 
lie engaged in the Ultramontane Controversy. He was 
for a time President of the Briinn Society^. 

For the photograph of Mendel which forms the frontis- 
piece to this work, I am indebted to the Very Rev. Dr 
Janeischek, the present Abbot of Briinn, who most kindly 
supplied it for this purpose. 

So far as I have discovered there was, up to 1900, only 
one reference to Mendel's observations in scientific literature, 
namely that of Focke, Pflanzenmischlinge , 1881, p. 109, 
* A few additional particulars are given in Tschermak's edition. 

Brief Historical Notice 37 

where it is simply stated that Mendel's numerous experi- 
ments on Pisum gave results similar to those obtained 
by Knight, but that he believed he had found constant 
numerical ratios among the types produced by hybridisation. 
In the same work a similar brief reference is made to the 
paper on Hieracium. 

It may seem surprising that a work of such importance 
should so long have failed to find recognition and to become 
current in the world of science. It is true that the journal 
in which it appeared is scarce, but this circumstance has 
seldom long delayed general recognition. The cause is 
unquestionably to be found in that neglect of the experi- 
mental study of the problem of Species which supervened 
on the general acceptance of the Darmnian doctrines. The 
problem of Species, as Kolreuter, Gartner, Naudin, Wichura, 
and the other hybridists of the middle of the nineteenth 
century conceived it, attracted thenceforth no workers. The 
question, it was imagined, had been answered and the 
debate ended. No one felt much interest in the matter. 
A host of other lines of work were suddenly opened up, and 
in 1865 the more original investigators naturally found 
those new methods of research more attractive than the 
tedious observations of the hybridisers, whose inquiries 
were supposed, moreover, to have led to no definite result. 

Nevertheless the total neglect of such a discovery is 
not easy to account for. Those who are acquainted with 
the literature of this branch of inquiry will know that the 
French Academy offered a prize in 1861 to be awarded in 
1862 on the subject ^^ Etudier les Hyhrides vegetaucc au 
point de vue de leur fecondite et de la perpetuite de leurs 
caracteresy This subject was doubtless chosen with 
reference to the experiments of Godron of Nancy and 
Naudin, then of Paris. Both these naturalists competed. 

38 Brief Historical Notice 

and the accounts of the work of Godron on Datura and 
of Nan din on a number of species were published in the 
years 1864 and 1865 respectively. Both, especially the 
latter, are works of high consequence in the history of the 
science of heredity. In the latter paper Naudin clearly 
enuntiated what we shall henceforth know as the Mendelian 
conception of the dissociation of characters of cross-breds 
in the formation of the germ-cells, though apparently he 
never developed this conception. 

In the year 1864, George Bentham, then President of 
the Linnean Society, took these treatises as the subject of 
his address to the Anniversary meeting on the 24 May, 
Naudin's work being known to him from an abstract, the 
full paper having not yet appeared. Referring to the 
hypothesis of dissociation which he fully described, he said 
that it appeared to be new and well supported, but required 
much more confirmation before it could be held as proven. 
{J. Linn. Soc, Bot., viii., Froc, p. xiv.) 

In 1865, the year of Mendel's communication to the 
Briinn Society, appeared Wichura's famous treatise on his 
experiments with Salicc to which Mendel refers. There are 
passages in this memoir which come very near Mendel's 
principles, but it is evident from the plan of his experiments 
that Mendel had conceived the whole of his ideas before 
that date. 

In 1868 appeared the first edition of Darwin's Animals 
and Plants, marking the very zenith of these studies, and 
thenceforth the decline in the experimental investigation 
of Evolution and the problem of Species has been steady. 
With the rediscovery and confirmation of Mendel's work 
by de Vries, Correns and Tschermak in 1900 a new era 

That Mendel's work, appearing, as it did, at a moment 

Brief Historical Notice 39 

when several naturalists of the first rank were still occupied 
with these problems, should have passed wholly unnoticed, 
wdll always remain inexplicable, the more so as the Briinn 
Society exchanged its publications with most of the 
Academies of Europe, including both the Royal and 
Linnean Societies. 

Nau din's views were well known to Darwin and are 
discussed in Animals and Plants (ed. 1885, ii., p. 23); but, 
put forward as they were without full proof, they could not 
command universal credence. Gartner, too, had adopted 
opposite views; and Wichura, working with cases of 
another order, had proved the fact that some hybrids breed 
true. Consequently it is not to be wondered at that 
Darwin was sceptical. Moreover, the Mendelian idea of 
the "hybrid-character," or heterozygous form, was unknown 
to him, a conception without which the hypothesis of dis- 
sociation of characters is quite imperfect. 

Had Mendel's work come into the hands of Darwin, it 
is not too much to say that the history of the development 
of evolutionary philosophy would have been very different 
from that which we have witnessed. 

By Gregor Mendel. 

{Read at the Meetings of the Sth February 
and 8th March, 1865.) 

Introductory Remarks. 

Experience of artificial fertilisation, such as is effected 
with ornamental plants in order to obtain new variations 
in colour, has led to the experiments which will here be 
discussed. The striking regularity with which the same 
hybrid forms always reappeared whenever fertilisation took 
place between the same species induced further experiments 
to be undertaken, the object of which was to follow up the 
developments of the hybrids in their progeny. 

To this object numerous careful observers, such as 
Kolreuter, Gartner, Herbert, Lecoq, Wichura and others, 
have devoted a part of their lives with inexhaustible 
perseverance. Gartner especially, in his work "Die Bas- 
tarderzeugung im Pflanzenreiche" (The Production of 
Hybrids in the Vegetable Kingdom), has recorded very 
valuable observations ; and quite recently Wichura published 
the results of some profound investigations into the hybrids 

* [This translation was made by the Eoyal Horticultural Society, 
and is reprinted with modifications and corrections, by permission. 
The original paper was published in the Verh. naturf. Ver. in Brunn, 
Abhandlungeii, iv. 1865, which appeared in 1866.] 

Menders Experiments in Hybridisation 41 

of the Willow. That, so far, no generally applicable law 
governing the formation and development of hybrids has 
been successfully formulated can hardly be wondered at by 
anyone who is acquainted with the extent of the task, and 
can appreciate the difficulties with which experiments of 
this class have to contend. A final decision can only be 
arrived at when we shall have before us the results of 
detailed experiments made on plants belonging to the most 
diverse orders. 

Those who survey the work done in this department 
will arrive at the conviction that among all the numerous 
experiments made, not one has been carried out to such an 
extent and in such a way as to make it possible to determine 
the number of different forms under which the offspring of 
hybrids appear, or to arrange these forms with certainty 
according to their separate generations, or to definitely 
ascertain their statistical relations*. 

It requires indeed some courage to undertake a labour 
of such far-reaching extent ; it appears, however, to be the 
only right way by which we can finally reach the solution 
of a question the importance of which cannot be over- 
estimated in connection with the history of the evolution 
of organic forms. 

The paper now presented records the results of such 
a detailed experiment. This experiment was practically 
confined to a small plant group, and is now, after eight 
years' pursuit, concluded in all essentials. Whether the 
plan upon which the separate experiments were conducted 
and carried out was the best suited to attain the desired 
end is left to the friendly decision of the reader. 

* [It is to the clear conception of these three primary necessities 
that the whole success of Mendel's work is due. So far as I know 
this conception was absolutely new iu his day.] 

42 Menders Experiments 

Selection of the Experimental Plants. 

The value and utility of any experiment are determined 
by the fitness of the material to the purpose for which it is 
used, and thus in the case before us it cannot be immaterial 
what plants are subjected to experiment and in what manner 
such experiments are conducted. 

The selection of the plant group which shall serve for 
experiments of this kind must be made with all possible 
care if it be desired to avoid from the outset every risk of 
questionable results. 

The experimental plants must necessarily — 

1. Possess constant differentiating characters. 

2. The hybrids of such plants must, during the 
flowering period, be protected from the influence of all 
foreign pollen, or be easily capable of such protection. 

The hybrids and their offspring should suffer no marked 
disturbance in their fertility in the successive generations. 

Accidental impregnation by foreign pollen, if it oc- 
curred during the experiments and were not recognized, 
would lead to entirely erroneous conclusions. Reduced 
fertility or entire sterility of certain forms, such as occurs in 
the offspring of many hybrids, would render the experiments 
very difficult or entirely frustrate them. In order to dis- 
cover the relations in which the hybrid forms stand towards 
each other and also towards their progenitors it appears to 
be necessary that all members of the series developed in 
each successive generation should be, without exception, 
subjected to observation. 

At the very outset special attention was devoted to the 
Leguminosce on account of their peculiar floral structure. 

in Hybridisation 43 

Experiments which were made with several members of this 
family led to the result that the genus Pisum was found to 
possess the necessary conditions. 

Some thoroughly distinct forms of this genus possess 
characters which are constant, and easily and certainly 
recognisable, and when their hybrids are mutually crossed 
they yield perfectly fertile progeny. Furthermore, a dis- 
turbance through foreign pollen cannot easily occur, since 
the fertilising organs are closely packed inside the keel and 
the anther bursts within the bud, so that the stigma 
becomes covered with pollen even before the flower opens. 
This circumstance is of especial importance. As additional 
advantages worth mentioning, there may be cited the easy 
culture of these plants in the open ground and in pots, and 
also their relatively short period of growth. Artificial 
fertilisation is certainly a somewhat elaborate process, but 
nearly always succeeds. For this purpose the bud is 
opened before it is perfectly developed, the keel is removed, 
and each stamen carefully extracted by means of forceps, 
after which the stigma can at once be dusted over with the 
foreign pollen. 

In all, thirty-four more or less distinct varieties of Peas 
were obtained from several seedsmen and subjected to a 
two years' trial. In the case of one variety there were 
remarked, among a larger number of plants all alike, a few 
forms which were markedly different. These, however, did 
not vary in the following year, and agreed entirely with 
another variety obtained from the same seedsmen ; the 
seeds were therefore doubtless merely accidentally mixed. 
All the other varieties yielded perfectly constant and 
similar offspring ; at any rate, no essential difference was 
observed during two trial years. For fertilisation twenty- 
two of these were selected and cultivated during the whole 

44 Mendel's Experiments 

period of the experiments. They remained constant without 
any exception. 

Their systematic classification is difficult and uncertain. 
If we adopt the strictest definition of a species, according 
to which only those individuals belong to a species which 
under precisely the same circumstances display precisely 
similar characters, no two of these varieties could be re- 
ferred to one species. According to the opinion of experts, 
however, the majority belong to the species Pisum satimim ; 
while the rest are regarded and classed, some as sub-species 
of P. sativum, and some as independent species, such as 
P. quadratum, P. saccharatum, and P. umhellatum. The 
positions, however, which may be assigned to them in a 
classificatory system are quite immaterial for the purposes 
of the experiments in question. It has so far been found 
to be just as impossible to draw a sharp line between the 
hybrids of species and varieties as between species and 
varieties themselves. 

Division and Arrangement of the Experiments. 

If two plants which differ constantly in one or several 
characters be crossed, numerous experiments have demon- 
strated that the common characters are transmitted un- 
changed to the hybrids and their progeny ; but each pair of 
differentiating characters, on the other hand, unite in the 
hybrid to form a new character, which in the progeny of the 
hybrid is usually variable. The object of the experiment 
was to observe these variations in the case of each pair of 
differentiating characters, and to deduce the law according 
to which they appear in the successive generations. The 
experiment resolves itself therefore into just as many 

in Hybridisation 45 

separate experiments as there are constantly differentiating 
characters presented in the experimental plants. 

The various forms of Peas selected for crossing showed 
difterences in the length and colour of the stem ; in the 
size and form of the leaves ; in the position, colour, and 
size of the flowers ; in the length of the flower stalk ; in the 
colour, form, and size of the pods ; in the form and size of 
the seeds ; and in the colour of the seed-coats and the 
albumen [cotyledons]. Some of the characters noted do 
not permit of a sharp and certain separation, since the 
difference is of a " more or less " nature, which is often 
difficult to define. Such characters could not be utilised 
for the separate experiments ; these could only be confined 
to characters which stand out clearly and definitely in the 
plants. Lastly, the result must show whether they, in 
their entirety, observe a regular behaviour in their hybrid 
unions, and whether fi'om these facts any conclusion can 
be come to regarding those characters which possess a 
subordinate significance in the type 

The characters which were selected for experiment relate : 

1. To the difference in the form of the ripe seeds. These 
are either round or roundish, the wrinkling, when such occurs 
on the surface, being always only shallow ; or they are 
irregularly angular and deeply wrinkled (P. quadratum). 

2. To the difference in the colour of the seed albumen 
(endosperm)*. The albumen of the ripe seeds is either 
pale yellow, bright yellow and orange coloured, or it 
possesses a more or less intense green tint. This diff"erence 
of colour is easily seen in the seeds as their coats are 

* [Mendel uses the terms "albumen" and "endosperm" somewhat 
loosely to denote the cotyledons, containing food-material, within the 

46 MendeVs Experiments 

3. To the difference in the colour of the seed-coat. 
Tliis is either white, with which character white flowers 
are constantly correlated ; or it is grey, grey-brown, leather- 
brown, with or without violet spotting, in which case the 
colour of the standards is violet, that of the wings purple, 
and the stem in the axils of the leaves is of a reddish tint. 
The grey seed-coats become dark brown in boiling water. 

4. To the difference in the form of the ripe pods. These 
are either simply inflated, never contracted in places ; or 
they are deeply constricted between the seeds and more or 
less wrinkled (P. saccharatum). 

5. To the difference in the colour of the unripe pods. 
They are either light to dark green, or vividly yellow, in 
which colouring the stalks, leaf-veins, and calyx par- 

6. To the difference in the position of the flowers. 
They are either axial, that is, distributed along the main 
stem ; or they are terminal, that is, bunched at the top of 
the stem and arranged almost in a false umbel ; in this 
case the upper part of the stem is more or less widened in 
section (P. umhellatun%)\ . 

7. To the difference in the length of the stem. The 
length of the stem J is very various in some forms; it is, 

* One species possesses a beautifully brownish-red coloured pod, 
which when ripening turns to violet and blue. Trials with this 
character were only begun last year. [Of these further experiments 
it seems no account was published. Correns has since worked with 
such a variety.] 

t [This is often called the Mummy Pea. It shows slight fasciation. 
The form I know has white standard and salmon-red wings.] 

X [In my account of these experiments {R.H.S. Journal, vol. xxv. 
p. 54) I misunderstood this paragraph and took " axis " to mean the 
floral axis, instead of the main axis of the plant. The unit of 
measurement, being indicated in the original by a dash ('), I care- 

in Hybridisation 47 

however, a constant character for each, in so far that healthy- 
plants, grown in the same soil, are only subject to unim- 
portant variations in this character. 

In experiments with this character, in order to be able to 
discriminate with certainty, the long axis of 6 — 7 ft. was 
always crossed with the short one of f ft. to Ij ft. 

Each two of the differentiating characters enumerated 
above were united by cross-fertilisation. There were made 

for the 

1st trial 60 fertilisations on 15 plants. 




55 10 




,5 10 




,5 10 




5, 5 




,5 10 




55 10 

From a larger number of plants of the same variety only 
the most vigorous were chosen for fertilisation. Weakly 
plants always afford uncertain results, because even in the 
first generation of hybrids, and still more so in the sub- 
sequent ones, many of the offspring either entirely fail to 
flower or only form a few and inferior seeds. 

Furthermore, in all the experiments reciprocal crossings 
were effected in such a way that each of the two varieties 
which in one set of fertilisations served as seed-bearers in 
the other set were used as pollen plants. 

The plants were grown in garden beds, a few also 
in pots, and were maintained in their naturally upright 
position by means of sticks, branches of trees, and strings 
stretched between. For each experiment a number of pot 
plants were placed during the blooming period in a green- 
house, to serve as control plants for the main experiment 

lessly took to have been an inch, but the translation here given is 
evidently correct.] 

48 MendeVs Experiments 

in the open as regards possible disturbance by insects. 
Among the insects* which visit Peas the beetle Bruchus 
pisi might be detrimental to the experiments should it 
appear in numbers. The female of this species is known 
to lay the eggs in the flower, and in so doing opens the 
keel ; upon the tarsi of one specimen, which was caught in 
a flower, some pollen grains could clearly be seen under a 
lens. Mention must also be made of a circumstance which 
possibly might lead to the introduction of foreign pollen. 
It occurs, for instance, in some rare cases that certain parts 
of an otherwise quite normally developed flower wither, 
resulting in a partial exposure of the fertilising organs. A 
defective development of the keel has also been observed, 
owing to which the stigma and anthers remained partially 
uncovered!. It also sometimes happens that the pollen 
does not reach full perfection. In this event there occurs 
a gradual lengthening of the pistil during the blooming 
period, until the stigmatic tip protrudes at the point of the 
keel. This remarkable appearance has also been observed 
in hybrids of Phaseolus and Lathyrus. 

The risk of false impregnation by foreign pollen is, 
however, a very slight one with Pisum, and is quite 
incapable of disturbing the general result. Among more 
than 10,000 plants which were carefully examined there 
were only a very few cases where an indubitable false 
impregnation had occurred. Since in the greenhouse such 
a case was never remarked, it may well be supposed that 
Bruchus pisi, and possibly also the described abnormalities 
in the floral structure, were to blame. 

* [It is somewhat surprising that no mention is made of Thrips, 
which swarm in Pea flowers. I had come to the conclusion that this 
is a real source of error and I see Laxton held the same opinion.] 

+ [This also happens in Sweet Peas.] 

in Hyhridisation 49 

The Forms of the Hybrids.* 

Experiments whicli in previous years were made witli 
ornamental plants have already afforded evidence that the 
hybrids, as a rule, are not exactly intermediate between 
the parental species. With some of the more striking 
characters, those, for instance, which relate to the form 
and size of the leaves, the pubescence of the several parts, 
&c., the intermediate, indeed, was nearly always to be 
seen ; in other cases, however, one of the two parental 
characters was so preponderant that it was difficult, or 
quite impossible, to detect the other in the hybrid. 

This is precisely the case with the Pea hybrids. In 
the case of each of the seven crosses the hybrid-character 
resembles! that of one of the parental forms so closely that 
the other either escapes observation completely or cannot 
be detected with certainty. This circumstance is of great 
importance in the determination and classification of the 
forms under which the offspring of the hybrids appear. 
Henceforth in this paper those characters which are trans- 
mitted entire, or almost unchanged in the hybridisation, 
and therefore in themselves constitute the characters of 
the hybrid, are termed the dominant, and those which 
become latent in the process recessive. The expression 
"recessive" has been chosen because the characters thereby 
designated withdraw or entirely disappear in the hybrids, 

* [Mendel throughout speaks of his cross-bred Peas as " hybrids," 
a term which many restrict to the offspring of two distinct species. 
He, as he explains, held this to be only a question of degree.] 

t [Note that Mendel, with true penetration, avoids speaking of 
the hybrid-character as "transmitted" by either parent, thus escap- 
ing the error pervading modern views of heredity.] 

B. 4 

50 MendeVs Experiments 

but nevertheless reappear unchanged in their progeny, as 
will be demonstrated later on. 

It was furthermore shown by the whole of the experi- 
ments that it is perfectly immaterial whether the dominant 
character belong to the seed-bearer or to the pollen parent ; 
the form of the hybrid remains identical in both cases. This 
interesting fact was also emphasised by Gartner, with the 
remark that je<5n ^tayinost practised expert is not in a 
position tOide^CTmine in\ hybrid which of the two parental 
species wm^e se^ orTheNpollen plant*. 

Of tha ^fferei^^tint gharacters which were used in the 
experimelffi tne fQiipwiifc ^i dominant : 

1. Tn^r&ind^r riJugngiish form of the seed with or 
without sA^ow d^^essiofes.1 

2. The^y^llowQoloumig of the seed albumen [coty- 
ledons], v^o '•' / 

3. The gr^y^J^sji^brown, or leather-brown colour of 
the seed-coat, in connection with violet-red blossoms and 
reddish spots in the leaf axils. 

4. The simply inflated form of the pod. 

5. The green colouring of the unripe pod in connection 
with the same colour in the stems, the leaf-veins and the calyx. 

6. The distribution of the flowers along the stem. 

7. The greater length of stem. 

"With regard to this last character it must be stated 
that the longer of the two parental stems is usually exceeded 
by the hybrid, which is possibly only attributable to the 
greater luxuriance which appears in all parts of plants 
when stems of very diff"erent length are crossed. Thus, for 
instance, in repeated experiments, stems of 1 ft. and 6 ft. 
in length yielded without exception hybrids which varied 
in length between 6 ft. and 7 J ft. 

* [Gartner, p. 223.] 

in Hybridisation 51 

The hybrid seeds in the experiments with seed-coat are 
often more spotted, and the spots sometimes coalesce into 
small bluish-violet patches. The spotting also frequently 
appears even when it is absent as a parental character. 

The hybrid forms of the seed-shape and of the albumen 
are developed immediately after the artificial fertilisation 
by the mere influence of the foreign pollen. They can, 
therefore, be observed even in the first year of experiment, 
whilst all the other characters naturally only appear in the 
following year in such plants as have been raised from the 
crossed seed. 

The First Generation [Bred] from the Hybrids. 

In this generation there reappear, together with the 
dominant characters, also the recessive ones with their full 
peculiarities, and this occurs in the definitely expressed 
average proportion of three to one, so that among each 
four plants of this generation three display the dominant 
character and one the recessive. This relates without 
exception to all the characters which were embraced in 
the experiments. The angular wrinkled form of the seed, 
the green colour of the albumen, the white colour of the 
seed-coats and the flowers, the constrictions of the pods, 
the yellow colour of the unripe pod, of the stalk of the 
calyx, and of the leaf venation, the umbel-like form of the 
inflorescence, and the dwarfed stem, all reappear in the 
numerical proportion given without any essential alteration. 
Transitional fm^ms were not observed in any experiment. 

Once the hybrids resulting from reciprocal crosses are 
fully formed, they present no appreciable difference in their 


52 MendeVs Expei^hnents 

subsequent development, and consequently the results [of 
the reciprocal crosses] can be reckoned together in each 
experiment. The relative numbers which were obtained for 
each pair of differentiating characters are as follows : 

Expt. 1. Form of seed. — From 253 hybrids 7,324 seeds 
were obtained in the second trial year. Among them were 
5,474 round or roundish ones and 1,850 angular wrinkled 
ones. Therefrom the ratio 2*96 to 1 is deduced. 

Expt. 2. Colour of albumen. — 258 plants yielded 8,023 
seeds, 6,022 yellow, and 2,001 green ; their ratio, therefore, 
is as 3*01 to 1. 

In these two experiments each pod yielded usually both 
kinds of seed. In well-developed pods which contained on 
the average six to nine seeds, it often occurred that all the 
seeds were round (Expt. 1) or all yellow (Expt. 2); on the 
other hand there were never observed more than five angular 
or five green ones in one pod. It appears to make no 
difference whether the pods are developed early or later in 
the hybrid or whether they spring from the main axis or 
from a lateral one. In some few plants only a few seeds 
developed in the first formed pods, and these possessed 
exclusively one of the two characters, but in the subse- 
quently developed pods the normal proportions were main- 
tained nevertheless. 

As in separate pods, so did the distribution of the 
characters vary in separate plants. By way of illustration 
the first ten individuals from both series of experiments 
may serve*. 

* [It is much to be regretted that Mendel does not give the 
complete series individually. No one who repeats such experiments 
should fail to record the individual numbers, which on seriation are 
sure to be full of interest.] 

in Hybridisation 53 

Experiment 1. 
Form of Seed. 


3riment 2. 
of Albumen. 




















































As extremes in the distribution of the two seed charac- 
ters in one plant, there were observed in Expt. 1 an instance 
of 43 round and only 2 angular, and another of 14 round 
and 15 angular seeds. In Expt. 2 there was a case of 32 
yellow and only 1 green seed, but also one of 20 yellow 
and 19 green. 

These two experiments are important for the determ- 
ination of the average ratios, because with a smaller 
number of experimental plants they show that very con- 
siderable fluctuations may occur. In counting the seeds, 
also, especially in Expt. 2, some care is requisite, since in 
some of the seeds of many plants the green colour of the 
albumen is less developed, and at first may be easily 
overlooked. The cause of the partial disappearance of the 
green colouring has no connection with the hybrid-character 
of the plants, as it likewise occurs in the parental variety. 
This peculiarity is also confined to the individual and is 
not inherited by the offspring. In luxuriant plants this 
appearance was frequently noted. Seeds which are damaged 
by insects during their development often vary in colour 
and form, but, w ith a little practice in sorting, errors are 

54 Menders Experiments 

easily avoided. It is almost superfluous to mention that the 
pods must remain on the plants until they are thoroughly 
ripened and have become dried, since it is only then that 
the shape and colour of the seed are fully developed. 

Expt. 3. Colour of the seed-coats. — Among 929 plants 
705 bore violet-red flowers and grey-brown seed-coats ; 224 
had white flowers and white seed-coats, giving the proportion 
3-15 to 1. 

Expt. 4. Form, of pods. — Of 1,181 plants 882 had them 
simply inflated, and in 299 they were constricted. Resulting 
ratio, 2"95 to 1. 

Expt. 5. Colour of the unripe pods. — The number of 
trial plants was 580, of which 428 had green pods and 152 
yellow ones. Consequently these stand in the ratio 2*82 to 1. 

Expt. 6. Position of flowers. — Among 858 cases 651 
blossoms were axial and 207 terminal. Ratio, 3*14 to 1. 

Expt. 7. Length of stem. — Out of 1,064 plants, in 787 
cases the stem was long, and in 277 short. Hence a mutual 
ratio of 2 "84 to 1. In this experiment the dwarfed plants 
were carefully lifted and transferred to a special bed. This 
precaution was necessary, as otherwise they would have 
perished through being overgrown by their tall relatives. 
Even in their quite young state they can be easily picked 
out by their compact growth and thick dark-green foliage. 

If now the results of the whole of the experiments be 
brought together, there is found, as between the number 
of forms with the dominant and recessive characters, an 
average ratio of 2 "98 to 1, or 3 to 1. 

The dominant character can have here a double signi- 
fication — viz. that of a parental-character, or a hybrid- 

in Hybridisation 55 

character*. In which of the two significations it appears 
in each separate case can only be determined by the follow- 
ing generation. As a parental character it must pass over 
unchanged to the whole of the offspring ; as a hybrid- 
character, on the other hand, it must observe the same 
behaviour as in the first generation. 

The Second Generation [Bred] from the Hybrids. 

Those forms which in the first generation maintain the 
recessive character do not further vary in the second 
generation as regards this character ; they remain constant 
in their offspring. 

It is otherwise with those which possess the dominant 
character in the first generation [bred from the hybrids]. 
Of these two-thirds yield offspring which display the 
dominant and recessive characters in the proportion of 
3 to 1, and thereby show exactly the same ratio as the 
hybrid forms, while only o??^- third remains with the domi- 
nant character constant. 

The separate experiments yielded the following results: — 

Expt. 1. — Among 565 plants which were raised from 
round seeds of the first generation, 193 yielded round seeds 
only, and remained therefore constant in this character ; 
372, however, gave both round and angular seeds, in the 
proportion of 3 to 1. The number of the hybrids, therefore, 
as compared with the constants is 1'93 to 1. 

Expt. 2. — Of 519 plants which were raised from seeds 
whose albumen was of yellow colour in the first generation, 
166 yielded exclusively yellow, while 353 yielded yellow 

* [This paragraph presents the view of the hybrid-character as 
something incidental to the hybrid, and not " transmitted " to it— a 
true and fundamental conception here expressed probably for the 
first time.] 

56 MendeVs Exj^eriments 

and green seeds in the proportion of 3 to 1. There resulted, 
therefore, a division into hybrid and constant forms in the 
proportion of 2 "13 to 1. 

For each separate trial in the following experiments 
100 plants were selected which displayed the dominant 
character in the first generation, and in order to ascertain 
the significance of this, ten seeds of each were cultivated. 

Expt. 3.— The offspring of 36 plants yielded exclusively 
grey-brown seed-coats, while of the offspring of 64 plants 
some had grey-brown and some had white. 

Expt. 4. — The offspring of 29 plants had only simply 
inflated pods ; of the offspring of 71, on the other hand, 
some had inflated and some constricted. 

Expt. 5. — The offspring of 40 plants had only green 
pods ; of the offspring of 60 plants some had green, some 
yellow ones. 

Expt. 6. — The offspring of 33 plants had only axial 
flowers ; of the offspring of 67, on the other hand, some 
had axial and some terminal flowers. 

Expt. 7. — The offspring of 28 plants inherited the long 
axis, and those of 72 plants some the long and some the 
short axis. 

In each of these experiments a certain number of the 
plants came constant with the dominant character. For 
the determination of the proportion in which the separation 
of the forms with the constantly persistent character results, 
the two first experiments are of especial importance, since 
in these a larger number of plants can be compared. The 
ratios 1*93 to 1 and 213 to 1 gave together almost exactly 
the average ratio of 2 to 1. The sixth experiment has a 
quite concordant result ; in the others the ratio varies more 
or less, as was only to be expected in view of the smaller 

i7b Hybridisation 57 

number of 100 trial plants. Experiment 5, which shows 
the greatest departure, was repeated, and then in lieu of 
the ratio of 60 and 40 that of 65 and 35 resulted. The 
average ratio of 2 to 1 appears, therefore, as fixed with 
certainty. It is therefore demonstrated that, of those forms 
which possess the dominant character in the first generation, 
in two-thirds the hybrid character is embodied, while one- 
third remains constant with the dominant character. 

The ratio of 3 to 1, in accordance with which the 
distribution of the dominant and recessive characters 
results in the first generation, resolves itself therefore in 
all experiments into the ratio of 2 : 1 : 1 if the dominant 
character be differentiated according to its significance as 
a hybrid character or a parental one. Since the members 
of the first generation spring directly from the seed of the 
hybrids, it is now dear that the hybrids form seeds having 
one or other of the two differentiating characters, and of 
these one-half develop again the hybrid form, while the other 
half yield plants which remain constant and receive the domi- 
nant or recessive characters [f^espectively] in equal numbers. 

The Subsequent Generations [Bred] from the Hybrids. 

The proportions in which the descendants of the hybrids 
develop and split up in the first and second generations 
presumably hold good for all subsequent progeny. Experi- 
ments 1 and 2 have already been carried through six 
generations, 3 and 7 through five, and 4, 5, and 6 through 
four, these experiments being continued from the third 
generation with a small number of plants, and no departure 
from the rule has been perceptible. The offspring of the 
hybrids separated in each generation in the ratio of 2 : 1 : 1 
into hybrids and constant forms. 


MendeVs Experiments 

If A be taken as denoting one of the two constant 
characters, for instance the dominant, a, the recessive, 
and A a the hybrid form in which both are conjoined, the 

A + 2Aa + a 

shows the terms in the series for the progenj^ of the hybrids 
of two differentiating characters. 

The observation made by Gartner, Kolreuter, and others, 
that hybrids are incHned to revert to the parental forms, is 
also confirmed by the experiments described. It is seen 
that the number of the hybrids which arise from one 
fertilisation, as compared with the number of forms which 
become constant, and their progeny from generation to 
generation, is continually diminishing, but that never- 
theless they could not entirely disappear. If an average 
equality of fertility in all plants in all generations be 
assumed, and if, furthermore, each hybrid forms seed of 
which one-half yields hybrids again, while the other half 
is constant to both characters in equal proportions, the 
ratio of numbers for the offspring in each generation is 
seen by the following summary, in which A and a denote 
again the two parental characters, and Aa the hybrid 
forms. For brevity's sake it may be assumed that each 
plant in each generation furnishes only 4 seeds. 





















: 2 



























in Hybridisation 59 

In the tenth generation, for instance, 2**- 1 = 1023. 
There result, therefore, in each 2,048 plants which arise in 
this generation 1,023 with the constant dominant character, 
1,023 with the recessive character, and only two hybrids. 

The Offspring of Hybrids in which Several 
Differentiating Characters are Associated. 

In the experiments above described plants were used 
which differed only in one essential character*. The next 
task consisted in ascertaining whether the law of develop- 
ment discovered in these applied to each pair of differen- 
tiating characters when several diverse characters are united 
in the hybrid by crossing. As regards the form of the 
hybrids in these cases, the experiments showed throughout 
that this invariably more nearly approaches to that one of 
the two parental plants which possesses the greater number 
of dominant characters. If, for instance, the seed plant has 
a short stem, terminal white flowers, and simply inflated 
pods ; the pollen plant, on the other hand, a long stem, 
violet-red flowers distributed along the stem, and con- 
stricted pods; the hybrid resembles the seed parent only in 
the form of the pod ; in the other characters it agrees with 
the pollen parent. Should one of the two parental types 
possess only dominant characters, then the hybrid is 
scarcely or not at all distinguishable from it. 

* [This statement of Mendel's in the light of present knowledge is 
open to some misconception. Though his work makes it evident that 
such varieties may exist, it is very unlikely that Mendel could have 
had seven pairs of varieties such that the members of each pair 
differed from each other in only one considerable character {loesentliches 
Merkmal). The point is probably of little theoretical or practical 
consequence, but a rather heavy stress is thrown on ^^ivesentlich.^'] 

60 Mendel's Experiments 

Two experiments were made with a larger number of 
plants. In the first experiment the parental plants differed 
in the form of the seed and in the colour of the albumen ; 
in the second in the form of the seed, in the colour of the 
albumen, and in the colour of the seed-coats. Experiments 
with seed characters give the result in the simplest and 
most certain way. 

In order to facilitate study of the data in these experi- 
ments, the different characters of the seed plant will be 
indicated hj A, B, C, those of the pollen plant by a, b, c, 
and the hybrid forms of the characters by A a, Bh, and Cc. 

Expt. 1. — J. ^, seed parents; a^, pollen parents ; 

A, form round ; a, form angular ; 

B, albumen yellow. b, albumen green. 

The fertilised seeds appeared round and yellow like those 
of the seed parents. The plants raised therefrom yielded 
seeds of four sorts, which frequently presented themselves 
in one pod. In all 556 seeds were yielded by 15 plants, 
and of these there were : — 

315 round and yellow, 
101 angular and yellow, 
108 round and green, 
32 angular and green. 

All were sown the following year. Eleven of the round 
yellow seeds did not yield plants, and three plants did not 
form seeds. Among the rest : 

38 had round yellow seeds . . . AB 

65 round yellow and green seeds . . ABb 

60 round yellow and angular yellow seeds AaB 
138 round yellow and green, angular yellow 

and green seeds . . . . ' AaBb. 

in Hybridisation 


From the angular yellow seeds 96 resulting plants bore 
seed, of which : 

28 had only angular yellow seeds aB 

68 angular yellow and green seeds aBh. 

From 108 round green seeds 102 resulting plants fruited, 
of which : 

35 had only round green seeds Ah 

67 round and angular green seeds Aab. 

The angular green seeds yielded 30 plants which bore seeds 
all of like character ; they remained constant ab. 

The offspring of the hybrids appeared therefore under 
nine different forms, some of them in very unequal numbers. 
When these are collected and co-ordinated we find : 

38 plants with the sign AB 




, aB 


, ab 








, Aah 



The whole of the forms may be classed into three 
essentially different groups. The first embraces those with 
the signs AB, Ah, aB, and ab : they possess only constant 
characters and do not vary again in the next generation. 
Each of these forms is represented on the average thirty- 
three times. The second group embraces the signs ABh, 
aBh, AaB, Aab : these are constant in one character and 
hybrid in another, and vary in the next generation only 
as regards the hybrid character. Each of these appears on 

62 MendeVs Experiments 

an average sixty-five times. The form AaBb occurs 138 
times : it is hybrid in both characters, and behaves exactly 
as do the hybrids from which it is derived. 

If the numbers in which the forms belonging to these 
classes appear be compared, the ratios of 1, 2, 4 are un- 
mistakably evident. The numbers 32, 65, 138 present very 
fair approximations to the ratio numbers of 33, 66, 132. 

The developmental series consists, therefore, of nine 
classes, of which four appear therein always once and are 
constant in both characters ; the forms AB, ah^ resemble 
the parental forms, the two others present combinations 
between the conjoined characters A, a, B, h, which com- 
binations are likewise possibly constant. Four classes 
appear always twice, and are constant in one character 
and hybrid in the other. One class appears four times, 
and is hybrid in both characters. Consequently the 
offspring of the hybrids, if two kinds of differentiating 
characters are combined therein, are represented by the 

AB+Ab + aB + ah+2ABh+2aBh + 2AaB+'lAah + 4.AaBh. 

This expression is indisputably a combination series in 
which the two expressions for the characters A and a, B 
and 6, are combined. We arrive at the full number of the 
classes of the series by the combination of the expres- 
sions : 

J. +2 Aa^a 

B+2 Bh + h. 
Second Expt. 

ABC, seed parents ; abc, pollen parents ; 

A, form round ; a, form angular ; 

B, albumen yellow ; h, albumen green ; 
G, seed-coat grey-brown, c, seed-coat white. 

in Hybridisation 


This experiment was made in precisely the same way as 
the previous one. Among all the experiments it demanded 
the most time and trouble. From 24 hybrids 687 seeds 
were obtained in all : these were all either spotted, grey- 
brown or grey- green, round or angular^. From these in 
the following year 639 plants fruited, and, as further 
investigation showed, there were among them : 

8 plants 



plants ABCc. 




14 „ 

A Be. 






9 „ 







11 » 







8 „ 







10 „ 







10 „ 





7 „ 




















The whole expression contains 27 terms. Of these 8 
are constant in all characters, and each appears on the 
average 10 times ; 12 are constant in two characters, and 
hybrid in the third; each appears on the average 19 times ; 
6 are constant in one character and hybrid in the other 
two ; each appears on the average 43 times. One form 
appears 78 times and is hybrid in all of the characters. 
The ratios 10, 19, 43, 78 agree so closely with the ratios 
10, 20, 40, 80, or 1, 2, 4, 8, that this last undoubtedly 
represents the true value. 

The development of the hybrids when the original 

* [Note that Mendel does not state the cotyledou-colour of the 
first crosses in this case ; for as the coats were thick, it could not 
have been seen without opening or peeling the seeds.] 

64 MendeVs Experiments 

parents differ in three characters results therefore according 
to the following expression : 

ABC + ABc + AhC + Abe + aBC + aBc + ahC + abc + 
2 ABCc + 2 AbCc + 2 aBCc + 2 abCc + 2 ABbC+ 2 ABbc + 
2 aBbC+ 2 aBbc + 2AaBC + 2 AaBc + 2 AabC+ 2 Aabc + 
4 ABbCc+A aBbCc + 4. AaBCc+4. AabCc + A AaBbC + 
4 AaBbc + 8 AaBbCc. 

Here also is involved a combination series in which the 
expressions for the characters A and a, B and b, C and c, 
are united. The expressions 

B-¥2Bb + b 

C+2CC + C 

give all the classes of the series. The constant combinations 
which occur therein agree with all combinations which are 
possible between the characters A, B, C,a,b,c\ two thereof, 
ABC and abc, resemble the two original parental stocks. 

In addition, further experiments were made with a 
smaller number of experimental plants in which the re- 
maining characters by twos and threes were united as 
hybrids : all yielded approximately the same results. There 
is therefore iw doubt that for the whole of the characters 
involved in the experiments the principle applies that the 
offspring of the hybrids in which several essentially different 
characters are combined represent the terms of a series of 
combinations, in which the developmental series for each pair 
of differentiating characters are associated. It is demon- 
strated at the same time that the relation of each pair of 
different characters in hybrid union is independent of the 
other differences in the two original parental stocks. 

If n represent the number of the differentiating charac- 
ters in the two original stocks, 3*^ gives the number of terms 

in Hyhridisation 65 

of the combination series, 4^^ the number of individuals 
which belong to the series, and 2"' the number of unions 
which remain constant. The series therefore embraces, if 
the original stocks differ in four characters, 3'* = 81 of classes, 
4'' =256 individuals, and 2^^=16 constant forms ; or, which 
is the same, among each 256 offspring of the hybrids there 
are 81 different combinations, 16 of which are constant. 

All constant combinations which in Peas are possible by 
the combination of the said seven differentiating characters 
were actually obtained by repeated crossing. Their number 
is given by 2'= 128. Thereby is simultaneously given the 
practical proof that the constant characters which appear in 
the several varieties of a group of plants may he obtained in 
all the associations which are possible according to the 
[mathematical] laws of combination^ by means of repeated 
artific ial fertilisation. 

As regards the flowering time of the hybrids, the ex- 
periments are not yet concluded. It can, however, already 
be stated that the period stands almost exactly between 
those of the seed and pollen parents, and that the con- 
stitution of the hybrids with respect to this character 
probably happens in the same way as in the case of the 
other characters. The forms which are selected for experi- 
ments of this class must have a difference of at least twenty 
days from the middle flowering period of one to that of the 
other; furthermore, the seeds when sown must all be placed 
at the same depth in the earth, so that they may germinate 
simultaneously. Also, during the whole flowering period, 
the more important variations in temperature must be taken 
into account, and the partial hastening or delaying of the 
flowering which may result therefrom. It is clear that this 
experiment presents many difficulties to be overcome and 
necessitates great attention. 

B. 5 

66 MendeVs Experiments 

If we endeavour to collate in a brief form the results 
arrived at, we find that those differentiating characters 
which admit of easy and certain recognition in the 
experimental plants, all behave exactly alike in their 
hybrid associations. The offspring of the hybrids of each 
pair of differentiating characters are, one-half, hybrid again, 
while the other half are constant in equal proportions having 
the characters of the seed and pollen parents respectively. 
If several differentiating characters are combined by cross- 
fertilisation in a hybrid, the resulting offspring form the 
terms of a combination series in which the permutation 
series for each pair of differentiating characters are united. 

The uniformity of behaviour shown by the whole of the 
characters submitted to experiment permits, and fully 
justifies, the acceptance of the principle that a similar 
relation exists in the other characters which appear less 
sharply defined in plants, and therefore could not be 
included in the separate experiments. An experiment 
with peduncles of different lengths gave on the whole a 
fairly satisfactory result, although the differentiation and 
serial arrangement of the forms could not be effected with 
that certainty which is indispensable for correct experiment. 

The Reproductive Cells of Hybrids. 

The results of the previously described experiments 
induced further experiments, the results of which appear 
fitted to afford some conclusions as regards the composition 
of the Qgg and pollen cells of hybrids. An important matter 
for consideration is afforded in Pisum by the circumstance 
that among the progeny of the hybrids constant forms 
appear, and that this occurs, too, in all combinations of the 
associated characters. So far as experience goes, we find 

in Hybridisation 67 

it in every case confirmed that constant progeny can only 
be formed when the egg cells and the fertilising pollen are 
of like character, so that both are provided with the material 
for creating quite similar individuals, as is the case with the 
normal fertilisation of pure species*. We must therefore 
regard it as essential that exactly similar factors are at work 
also in the production of the constant forms in the hybrid 
plants. Since the various constant forms are produced in 
one plant, or even in one flower of a plant, the conclusion 
appears logical that in the ovaries of the hybrids there are 
formed ,as many sorts of egg cells, and in the anthers as 
many sorts of pollen cells, as there are possible constant 
combination forms, and that these egg and pollen cells 
agree in their internal composition mth those of the 
separate forms. 

In point of fact it is possible to demonstrate theoretically 
that this hypothesis would fully sufiice to account for the 
development of the hybrids in the separate generations, if 
we might at the same time assume that the various kinds 
of egg and pollen cells were formed in the hybrids on the 
average in equal numbers!. 

In order to bring these assumptions to an experimental 
proof, the following experiments were designed. Two forms 
which were constantly difterent in the form of the seed and 
the colour of the albumen were united by fertilisation. 

If the differentiating characters are again indicated as 
A, B, a, b, we have : 

AB, seed parent ; ab, pollen parent ; 

A, form round; a, form angular; 

B, albumen yellow. b, albumen green. 

* [" False hybridism " was of course unknown to Mendel.] 
t [This and the preceding paragraph contain the essence of the 
Mendeliau principles of heredity.] 


68 Mendel's Experiments 

The artificially fertilised seeds were sown together with 
several seeds of both original stocks, and the most vigorous 
examples were chosen for the reciprocal crossing. There 
were fertilised : 

1. The hybrids with the pollen of AB. 

2. The hybrids ,, „ ab. 

3. AB „ „ the hybrids. 

4. ab „ „ the hybrids. 

For each of these four experiments the whole of the 
flowers on three plants were fertilised. If the above theory 
be correct, there must be developed on the hybrids egg and 
pollen cells of the forms AB, Ab, aB, ab, and there would 
be combined : — 

1. The Qgg cells AB, Ab, aB, ab with the pollen 
cells AB. 

2. The Qgg cells AB, Ab, aB, ab with the pollen 
cells ab. 

3. The Qgg cells AB with the pollen cells AB, Ab, 
aB, ab. 

4. The Qgg cells ab with the pollen cells AB, Aby 
aB, ab. 

From each of these experiments there could then result 
only the following forms : — 

1. AB, ABb, AaB, AaBb. 

2. AaBb, Aab, aBb, ab. 

3. AB, ABb, AaB, AaBb. 

4. AaBb, Aab, aBb, ab. 

If, furthermore, the several forms of the Qgg and pollen 
cells of the hybrids were produced on an average in equal 
numbers, then in each experiment the said four combinations 

in Hyhridisation 69 

should stand in the same ratio to each other. A perfect 
agreement in the numerical relations was, however, not to 
be expected, since in each fertilisation, even in normal 
cases, some &gg cells remain undeveloped or subsequently 
die, and many even of the well-formed seeds fail to 
germinate when sown. The above assumption is also 
limited in so far that, while it demands the formation of 
an equal number of the various sorts of Qgg and pollen 
cells, it does not require that this should apply to each 
separate hybrid with mathematical exactness. 

The first and second experiments had primarily the 
object of proving the composition of the hybrid Qgg cells, 
while the third and fourth experiments were to decide that of 
the pollen cells*. As is shown by the above demonstration 
the first and second experiments and the third and fourth 
experiments should produce precisely the same combinations, 
and even in the second year the result should be partially 
visible in the form and colour of the artificially fertilised 
seed. In the first and third experiments the dominant 
characters of form and colour, A and B, appear in each 
union, and are also partly constant and partly in hybrid 
union with the recessive characters a and h, for which 
reason they must impress their peculiarity upon the whole 
of the seeds. All seeds should therefore appear round and 
yellow, if the theory be justified. In the second and fourth 
experiments, on the other hand, one union is hybrid in 
form and in colour, and consequently the seeds are round 
and yellow ; another is hybrid in form, but constant in the 
recessive character of colour, whence the seeds are round 
and green ; the third is constant in the recessive character 
of form but hybrid in colour, consequently the seeds are 

* [To prove, namely, that both were similarly differentiated, and 
not one or other only.] 

70 MeyideVs Experiments 

angular and yellow ; the fourth is constant in both recessive 
characters, so that the seeds are angular and green. In 
both these experiments there were consequently four sorts 
of seed to be expected — viz. round and yellow, round and 
green, angular and yellow, angular and green. 

The crop fulfilled these expectations perfectly. There 
were obtained in the 

1st Experiment, 98 exclusively round yellow seeds ; 
dru ,, y4: ,, ' jj J) j> 

In the 2nd Experiment, 31 round and yellow, 26 round 
and green, 27 angular and yellow, 26 angular and green seeds. 

In the 4th Experiment, 24 round and yellow, 25 round 
and green, 22 angular and yellow, 27 angular and green 

A favourable result could now scarcely be doubted ; the 
next generation must afford the final proof From the seed 
sown there resulted for the first experiment 90 plants, and 
for the third 87 plants which fruited : these yielded for the — 

1st Exp. 3rd Exp. 

20 25 round yellow seeds . . . . . AB 

23 19 round yellow and green seeds . . ABh 

25 22 round and angular yellow seeds . . AaB 

22 21 round and angular green and yellow seeds AaBb 

In the second and fourth experiments the round and 
yellow seeds yielded plants with round and angular yellow 
and green seeds, AaBb. 

From the round green seeds plants resulted with round 
and angular green seeds, Aab. 

The angular yellow seeds gave plants with angular 
yellow and green seeds, aBb. 

From the angular green seeds plants were raised which 
yielded again only angular and green seeds, ab. 

in Hybridisation 71 

Although in these two experiments likewise some seeds 
did not germinate, the figures arrived at already in the 
previous year were not affected thereby, since each kind of 
seed gave plants which, as regards their seed, were like each 
other and different from the others. There resulted there- 
fore from the 

2nd Exp. 

4tli Exp. 



plants of the form AaBh 



„ „ Aab 



„ „ aBb 




In all the experiments, therefore, there appeared all the 
forms which the proposed theory demands, and also in 
nearly equal numbers. 

In a further experiment the characters of floral colour 
and length of stem were experimented upon, and selection 
so made that in the third year of the experiment each 
character ought to appear in half of all the plants if the 
above theory were correct. A, B, a, h serve again as 
indicating the various characters. 

J., violet-red flowers. a, white flowers. 

B, axis long. h, axis short. 

The form Ah was fertilised with ah, which produced the 
hybrid Aah. Furthermore, aB was also fertilised with ah, 
whence the hybrid aBh. In the second year, for further 
fertilisation, the hybrid Aah was used as seed parent, and 
hybrid aBh as pollen parent. 

Seed parent, Aah. Pollen parent, aBh. 

Possible Qgg cells, Ahah. Pollen cells, aBah. 

From the fertilisation between the possible egg and 
pollen cells four combinations should result, viz. : — 
AaBh + aBb + Aah + ah. 

7^ MendeVs Experiments 

From this it is perceived that, according to the above 
theory, in the third year of the experiment out of all the 

Half should have violet-red flowers {Aa), Classes 1, 3 

„ „ ,, white flowers (a) ,, 2, 4 

„ „ „ a long axis {Bh) „ 1, 2 

„ „ „ a short axis (b) „ 3, 4 

From 45 fertilisations of the second year 187 seeds 
resulted, of which only 166 reached the flowering stage in 
the third year. Among these the separate classes appeared 
in the numbers following : — 


Colour of flower. 





47 times 




40 „ 




38 „ 




41 „ 

There consequently appeared — 

The violet-red flower colour (Aa) in 85 plants. 
„ white „ „ (a) in 81 „ 

„ long stem (Bb) in 87 „ 

„ short „ (6) in 79 „ 

The theory adduced is therefore satisfactorily confirmed in 
this experiment also. 

For the characters of form of pod, colour of pod, and 
position of flowers experiments were also made on a small 
scale, and results obtained in perfect agreement. All 
combinations which were possible through the union of the 
differentiating characters duly appeared, and in nearly 
equal numbers. 

Experimentally, therefore, the theory is justified that 
the pea hybrids form egg and pollen cells which, in their 

in Hybridisation 73 

constitution^ represent in equal numbers all constant forms 
which result from the combination of the characters when 
united in fertilisation. 

The difference of the forms among the progeny of the 
hybrids, as well as the respective ratios of the numbers in 
which they are observed, find a sufficient explanation in the 
principle above deduced. The simplest case is afforded by 
the developmental series of each pair of differentiating 
characters. This series is represented by the expression 
A + 2Aa + a, in which A and a signify the forms with 
constant differentiating characters, and Aa the hybrid 
form of both. It includes in three different classes four 
individuals. In the formation of these, pollen and Qgg 
cells of the form A and a take part on the average equally 
in the fertilisation ; hence each form [occurs] twice, since 
four individuals are formed. There participate consequently 
in the fertilisation — 

The poUen cells A + A-^ a + a 
The Qgg cells A + A + a + a. 

It remains, therefore, purely a matter of chance which 
of the two sorts of pollen will become united with each 
separate Qgg cell. According, however, to the law of 
probability, it will always happen, on the average of many 
cases, that each pollen form A and a will unite equally 
often with each Qgg cell form A and «, consequently one 
of the two pollen cells A in the fertilisation will meet with 
the Qgg cell A and the other with an Qgg cell a, and so 
likewise one pollen cell a will unite with an Qgg cell A^ 
and the other with egg cell a. 

Pollen cells A A a a 



Egg cells A A a a 

74 MendeVs Experiments 

The result of the fertilisation may be made clear by 
putting the signs for the conjoined Qgg and pollen cells in 
the form of fractions, those for the pollen cells above and 
those for the Qgg cells below the line. We then have 

A A a a 

— + — +— +-. 
A a A a 

In the first and fourth term the Qgg and pollen cells are of 
like kind, consequently the product of their union must be 
constant, viz. A and a\ in the second and third, on the 
other hand, there again results a union of the two different- 
iating characters of the stocks, consequently the forms 
resulting from these fertilisations are identical with those 
of the hybrid from which they sprang. There occurs 
accordingly a repeated hybridisation. This explains the 
striking fact that the hybrids are able to produce, besides 
the two parental forms, offspring which are like themselves ; 

— and -r both give the same union Aa, since, as already 
a A 

remarked above, it makes no difference in the result of 
fertilisation to which of the two characters the pollen or 
egg cells belong. We may write then — 

A A a a . ^ . 

— + — +-r+- = J. + 2Aa + a. 
A a A a 

This represents the average result of the self-fertilisation 
of the hybrids when two differentiating characters are 
united in them. In solitary flowers and in solitary plants, 
however, the ratios in which the forms of the series are pro- 
duced may suffer not inconsiderable fluctuations*. Apart 
from the fact that the numbers in which both sorts of Qgg 
cells occur in the seed vessels can only be regarded as equal 
on the average, it remains purely a matter of chance which 

* [Whether segregation by such units is more than purely for- 
tuitous could probably be determined by seriation.] 

in Hybridisation 75 

of the two sorts of pollen may fertilise each separate egg 
cell. For this reason the separate values must necessarily 
be subject to fluctuations, and there are even extreme cases 
possible, as were described earlier in connection with the 
experiments on the form of the seed and the colour of the 
albumen. The true ratios of the numbers can only be 
ascertained by an average deduced from the sum of as 
many single values as possible ; the greater the number 
the more are merely chance elements eliminated. 

The developmental series for hybrids in which two 
kinds of differentiating characters are united contains 
among sixteen individuals nine different forms, viz., 
AB + Ab + aB + ab + 2ABb+2aBb + 2AaB+2Aab+4.AaBb. 
Between the differentiating characters of the original stocks 
Aa and Bb four constant combinations are possible, and 
consequently the hybrids produce the corresponding four 
forms of egg and pollen cells AB, Ab, aB, ab, and each 
of these will on the average figure four times in the 
fertilisation, since sixteen individuals are included in the 
series. Therefore the participators in the fertilisation are — 
Pollen cells AB + AB + AB + AB + Ab + Ab + Ab + Ab + 

aB + aB + aB + aB + ab + ab + ab + ab. 
Egg cells AB + AB + AB + AB + Ab + Ab + Ab + Ab + 

aB + aB + aB + aB + ab + ab + ab + ab. 
In the process of fertilisation each pollen form unites on an 
average equally often with each egg cell form, so that each 
of the four pollen cells AB unites once with one of the 
forms of egg cell AB, Ab, aB, ab. In precisely the same 
way the rest of the pollen cells of the forms Ab, aB, ab 
unite with all the other egg cells. We obtain therefore — 
AB AB AB AB Ah Ab Ab Ab 
AB ^ Ab^ aB "^ ab ^ AB^ Ab^ aB^ ab^ 
aB aB aB aB ab ab ab ab 

7Q Mender s Experiments 


AB + ABh + AaB + AaBh + ABh + Ah ■¥ AaBb + Aah + 
AaB^- AaBh + aB + ^56 + A aBh + J^a^ + aBh -^ah = AB + 
Ah + aB + ah + 2 ABh + 2aBh + 2^a5 + 2 Aah + 4.AaBh^. 

In precisely similar fashion is the developmental series 
of hybrids exhibited when three kinds of differentiating 
characters are conjoined in them. The hybrids form eight 
various kinds of egg and pollen cells — ABC, ABc, AhC, 
Abe, aBC, aBc, ahC, ahc — and each pollen form unites 
itself again on the average once with each form of Qgg cell. 

The law of combination of different characters which 
governs the development of the hybrids finds therefore its 
foundation and explanation in the principle enunciated, 
that the hybrids produce egg cells and pollen cells which 
in equal numbers represent all constant forms which result 
from the combinations of the characters brought together 
in fertilisation. 

Experiments with Hybrids of other Species of Plants. 

It must be the object of further experiments to ascertain 
whether the law of development discovered for Pisum 
applies also to the hybrids of other plants. To this end 
several experiments were recently commenced. Two minor 
experiments with species of Phaseolus have been completed, 
and may be here mentioned. 

An experiment with Phaseolus vulgaris and Phaseolus 
nanus gave results in perfect agreement. Ph. nanus had 
together with the dwarf axis simply inflated green pods. 
Ph. vulgaris had, on the other hand, an axis 10 feet to 

* [In the original the sign of equality ( = ) is here represented 
by + , evidently a misprint.] 

in Hybridisation 77 

12 feet high, and yellow coloured pods, constricted when 
ripe. The ratios of the numbers in which the different 
forms appeared in the separate generations were the same 
as with Pisum. Also the development of the constant 
combinations resulted according to the law of simple com- 
bination of characters, exactly as in the case of Pisum. 
There were obtained — 



Colour of 

Form of 


the unripe pods. 

the ripe pods. 

































The green colour of the pod, the inflated forms, and the 
long axis were, as in Pisum, dominant characters. 

Another experiment with two very different species of 
Phaseolus had only a partial result. Phaseolus nanus, L., 
served as seed parent, a perfectly constant species, with 
white flowers in short racemes and small white seeds in 
straight, inflated, smooth pods; as pollen parent was used 
Ph. multiflorus, W., with tall winding stem, purple-red 
flowers in very long racemes, rough, sickle-shaped crooked 
pods, and large seeds which bore black flecks and splashes 
on a peach-blood-red ground. 

The hybrids had the greatest similarity to the pollen 
parent, but the flowers appeared less intensely coloured. 
Their fertility was very limited; from seventeen plants, 
which together developed many hundreds of flowers, only 
forty-nine seeds in all were obtained. These were of 

78 MendeVs Experiments 

medium size, and were flecked and splashed similarly to 
those of Ph. muUiflorus, while the ground colour was not 
materially different. The next year forty-four plants were 
raised from these seeds, of which only thirty-one reached 
the flowering stage. The characters of Ph. nanus, which 
had been altogether latent in the hybrids, reappeared in 
various combinations; their ratio, however, with relation 
to the dominant characters was necessarily very fluctuating 
owing to the small number of trial plants. With certain 
characters, as in those of the axis aud the form of pod, it 
was, however, as in the case of Pisum, almost exactly 1:3. 
Insignificant as the results of this experiment may be as 
regards the determination of the relative numbers in which 
the various forms appeared, it presents, on the other hand, 
the phenomenon of a remarkable change of colour in the 
flowers and seed of the hybrids. In Pisum it is known 
that the characters of the flower- and seed-colour present 
themselves unchanged in the first and further generations, 
and that the offspring of the hybrids display exclusively the 
one or the other of the characters of the original stocks'^. 
It is otherwise in the experiment we are considering. The 
white flowers and the seed-colour of Ph. nanus appeared, it 
is true, at once in the first generation [/rom the hybrids] 
in one fairly fertile example, but the remaining thirty 
plants developed flower colours which were of various 
grades of purple-red to pale violet. The colouring of the 
seed-coat was no less varied than that of the flowers. No 

* [This is the only passage where Mendel can be construed as 
asserting universal dominance for Pisum ; and even here, having 
regard to the rest of the paper, it is clearly unfair to represent him as 
predicating more than he had seen in his own experiments. More- 
over in flower and seed-coat colour (which is here meant), using his 
characters dominance must be almost universal, if not quite.] 

iri Hybridisation 79 

plant could rank as fully fertile; many produced no fruit 
at all ; others only yielded fruits from the flowers last pro- 
duced, which did not ripen. From fifteen plants only were 
well-developed seeds obtained. The greatest disposition 
to infertility was seen in the forms with preponderantly 
red flowers, since out of sixteen of these only four yielded 
ripe seed. Three of these had a similar seed pattern to 
Pk. multiflomis, but with a more or less pale ground colour; 
the fourth plant yielded only one seed of plain brown tint. 
The forms with preponderantly violet coloured flowers had 
dark brown, black-brown, and quite black seeds. 

The experiment was continued through two more genera- 
tions under similar unfavourable circumstances, since even 
among the offspring of fairly fertile plants there were still 
some which were less fertile or even quite sterile. Other 
flower- and seed-colours than those cited did not sub- 
sequently present themselves. The forms w^hich in the 
first generation [bred from the hybrids] contained one or 
more of the recessive characters remained, as regards these, 
constant without exception. Also of those plants which 
possessed violet flowers and brown or black seed, some did 
not vary again in these respects in the next generation; 
the majority, however, yielded, together with ofi"spring 
exactly like themselves, some which displayed white flowers 
and white seed-coats. The red flowering plants remained 
so slightly fertile that nothing can be said with certainty 
as regards their further development. 

Despite the many disturbing factors with which the 
observations had to contend, it is nevertheless seen by this 
experiment that the development of the hybrids, with 
regard to those characters which concern the form of the 
plants, follows the same laws as does Pisum. With regard 
to the colour characters, it certainly appears difficult to 

80 MendeVs Experiments 

perceive a substantial agreement. Apart from the fact 
that from the union of a white and a purple-red colouring 
a whole series of colours results, from purple to pale violet 
and white, the circumstance is a striking one that among 
thirty-one flowering plants only one received the recessive 
character of the white colour, while in Pisum this occurs 
on the average in every fourth plant. 

Even these enigmatical results, however, might probably 
be explained by the law governing Pisum if we might 
assume that the colour of the flowers and seeds of Ph. 
multiflorus is a combination of two or more entirely 
independent colours, which individually act like any other 
constant character in the plant. If the flower colour A 
were a combination of the individual characters A^ + A^+ ... 
which produce the total impression of a purple colouration, 
then by fertilisation with the differentiating character, 
white colour, a, there would be produced the hybrid unions 
A^a + A^a + ... and so would it be with the corresponding 
colouring of the seed-coats'*. According to the above 
assumption, each of these hybrid colour unions would be 
independent, and would consequently develop quite inde- 
pendently from the others. It is then easily seen that 
from the combination of the separate developmental series 

* [It appears to me clear that this expression is incorrectly given, 
and the argument regarding compound characters is consequently not 
legitimately developed. The original compound character should be 
represented as A-^A^A^... which when fertilised by a-^ gives A-j^A^A^..^ 
as the hybrid of the first generation. Mendel practically tells us 
these were all alike, and there is nothing to suggest that they were 
diverse. When on self-fertilisation, they break up, they will produce 
the gametes he specifies ; but they may also produce A-^A-^ and A^A^y 
A^A^a, &G., thereby introducing terms of a nature different from any 
indicated by him. That this point is one of the highest significance, 
both practical and theoretical, is evident at once. ] 

in Hybridisation 81 

a perfect colour-series must result. If, for instance, 
A = A^+ A^, then the hybrids A^a and A^a form the 
developmental series — 

Ai + 2A^a + a 
A^+ 2A^a-¥a. 

The members of this series can enter into nine different 
combinations, and each of these denotes another colour"^ — 

1 A^A^ 2 A^aA^ 1 A^a 

2 A^A^a 4 A^aA^a 2 A^aa 
1 A^a 2 Aiaa 1 aa. 

The figures prescribed for the separate combinations 
also indicate how many plants with the corresponding 
colouring belong to the series. Since the total is sixteen, 
the whole of the colours are on the average distributed 
over each sixteen plants, but, as the series itself indicates, 
in unequal proportions. 

Should the colour development really happen in this 
way, we could offer an explanation of the case above 
described, viz. that the white flowers and seed-coat colour 
only appeared once among thirty-one plants of the first 
generation. This colouring appears only once in the series, 
and could therefore also only be developed once in the 
average in each sixteen, and with three colour characters 
only once even in sixty-four plants. 

It must, however, not be forgotten that the explanation 
here attempted is based on a mere hypothesis, only supported 
by the very imperfect result of the experiment just de- 
scribed. It would, however, be well worth while to follow 
up the development of colour in hybrids by similar experi- 

* [It seems very doubtful if the zygotes are correctly represented by 
the terms A^aA^a, A/xa, A-^^aa ; for in the hybrids A^a, &c. the allelo- 
morphs Ai and a, &c. should by hypothesis be separated in the gametes.] 

B. 6 

82 Mender s Experiments 

ments, since it is probable that in this way we might learn 
the significance of the extraordinary variety in the colouring 
of our ornamental flowers. 

So far, little at present is known with certainty beyond 
the fact that the colour of the flowers in most ornamental 
plants is an extremely variable character. The opinion has 
often been expressed that the stability of the species is 
greatly disturbed or entirely upset by cultivation, and 
consequently there is an inclination to regard the develop- 
ment of cultivated forms as a matter of chance devoid of 
rules ; the colouring of ornamental plants is indeed usually 
cited as an example of great instability. It is, however, 
not clear why the simple transference into garden soil 
should result in such a thorough and persistent revolution 
in the plant organism. No one will seriously maintain 
that in the open country the development of plants is ruled 
by other laws than in the garden bed. Here, as there, 
changes of type must take place if the conditions of life be 
altered, and the species possesses the capacity of fitting 
itself to its new environment. It is willingly granted that 
by cultivation the origination of new varieties is favoured, 
and that by man's labour many varieties are acquired 
which, under natural conditions, would be lost ; but nothing 
justifies the assumption that the tendency to the formation 
of varieties is so extraordinarily increased that the species 
speedily lose all stability, and their offspring diverge into 
an endless series of extremely variable forms. Were the 
change in the conditions of vegetation the sole cause of 
variability we might expect that those cultivated plants 
which are grown for centuries under almost identical con- 
ditions would again attain constancy. That, as is well 
known, is not the case, since it is precisely under such 
circumstances that not only the most varied but also the 

in Hybridisation 83 

most variable forms are found. It is only the Leguminosce , 
like Pisum, Phaseolus, Lens, whose organs of fertilisation 
are protected by the keel, which constitute a noteworthy 
exception. Even here there have arisen numerous varieties 
during a cultural period of more than 1000 years ; these 
maintain, however, under unchanging environments a sta- 
bility as great as that of species growing wild. 

It is more than probable that as regards the variability 
of cultivated plants there exists a factor which so far has 
received little attention. Various experiments force us to 
the conclusion that our cultivated plants, with few ex- 
ceptions, are members of various hybrid series, whose 
further development in conformity with law is changed and 
hindered by frequent crossings inter se. The circumstance 
must not be overlooked that cultivated plants are mostly 
grown in great numbers and close together, affording 
the most favourable conditions for reciprocal fertilisation 
between the varieties present and the species itself The 
probability of this is supported by the fact that among the 
great array of variable forms solitary examples are always 
found, which in one character or another remain constant, 
if only foreign influence be carefully excluded. These forms 
develop precisely as do those which are known to be members 
of the compound hybrid series. Also with the most sus- 
ceptible of all characters, that of colour, it cannot escape 
the careful observer that in the separate forms the inclination 
to vary is disj^layed in very different degrees. Among 
plants which arise from one spontaneous fertilisation there 
are often some whose offspring vary widely in the constitution 
and arrangement of the colours, while others furnish forms of 
little deviation, and among a greater number solitary examples 
occur which transmit the colour of the flowers unchanged 
to their offspring. The cultivated species of Dianthus 


84 Menders Experiments 

afford an instructive example of this. A white-flowered 
example of Diantkus caryophyllus, which itself was derived 
from a white-flowered variety, was shut up during its 
blooming period in a greenhouse ; the numerous seeds 
obtained therefrom yielded plants entirely white-flowered 
like itself A similar result was obtained from a subspecies, 
with red flowers somewhat flushed with violet, and one 
with flowers white, striped with red. Many others, on the 
other hand, which were similarly protected, yielded progeny 
which were more or less variously coloured and marked. 

Whoever studies the colouration which results in orna- 
mental plants from similar fertilisation can hardly escape 
the conviction that here also the development follows a 
definite law which possibly finds its expression in the 
combination of several independent colour characters. 

Concluding Remarks. 

It can hardly fail to be of interest to compare the 
observations made regarding Pisum with the results arrived 
at by the two authorities in this branch of knowledge, 
Kolreuter and Gartner, in their investigations. According 
to the opinion of both, the hybrids in outer appearance 
present either a form intermediate between the original 
species, or they closely resemble either the one or the other 
type, and sometimes can hardly be discriminated from it. 
From their seeds usually arise, if the fertilisation was 
effected by their own pollen, various forms which differ 
from the normal type. As a rule, the majority of individuals 
obtained by one fertilisation maintain the hybrid form, 
while some few others come more like the seed parent, 
and one or other individual approaches the pollen parent. 
This, however, is not the case with all hybrids without 
exception. With some the offspring have more nearly 

in Hyhridisation 85 

approached, some the one and some the other, original 
stock, or they all incline more to one or the other side ; 
while with others they remain perfectly like the hybrid and 
continue constant in their offspring. The hybrids of varieties 
behave like hybrids of species, but they possess greater varia- 
bility of form and a more pronounced tendency to revert to 
the original type. 

With regard to the form of the hybrids and their 
development, as a rule an agreement with the observations 
made in Pisum is unmistakable. It is otherwise with the 
exceptional cases cited. Gartner confesses even that the 
exact determination whether a form bears a greater resem- 
blance to one or to the other of the two original species 
often involved great difficulty, so much depending upon 
the subjective point of view of the observer. Another 
circumstance could, however, contribute to render the 
results fluctuating and uncertain, despite the most careful 
observation and differentiation; for the experiments plants 
were mostly used which rank as good species and are 
differentiated by a large number of characters. In addition 
to the sharply defined characters, where it is a question of 
greater or less similarity, those characters must also be 
taken into account which are often difficult to define in 
words, but yet suffice, as every plant specialist knows, to 
give the forms a strange appearance. If it be accepted 
that the development of hybrids follows the law which is 
valid for Pisum, the series in each separate experiment 
must embrace very many forms, since the number of the 
components, as is known, increases with the number of 
the differentiating characters in cubic ratio. With a 
relatively small number of experimental-plants the result 
therefore could only be approximately right, and in single 
cases might fluctuate considerably. If, for instance, the 

86 MendeVs Experiments 

two original stocks differ in seven characters, and 100 and 
200 plants were raised from the seeds of their hybrids to 
determine the grade of relationship of the offspring, we can 
easily see how uncertain the decision must become, since 
for seven differentiating characters the combination series 
contains 16,384 individuals under 2187 various forms ; 
now one and then another relationship could assert its 
predominance, just according as chance presented this or 
that form to the observer in a majority of cases. 

If, furthermore, there appear among the differentiating 
characters at the same time dominant characters, which 
are transferred entire or nearly unchanged to the hybrids, 
then in the terms of the developmental series that one of 
the two original stocks which possesses the majority of 
dominant characters must always be predominant. In the 
experiment described relative to Pisum, in which three 
kinds of differentiating characters were concerned, all the 
dominant characters belonged to the seed parent. Although 
the terms of the series in their internal composition 
approach both original stock plants equally, in this experi- 
ment the type of the seed parent obtained so great a 
preponderance that out of each sixty-four plants of the 
first generation fifty-four exactly resembled it, or only 
differed in one character. It is seen how rash it may be 
under such circumstances to draw from the external resem- 
blances of hybrids conclusions as to their internal nature. 

Gartner mentions that in those cases where the develop- 
ment was regular among the offspring of the hybrids the 
two original species were not reproduced, but only a few 
closely approximating individuals. With very extended 
developmental series it could not in fact be otherwise. 
For seven differentiating characters, for instance, among 
more than 16,000 individuals — offspring of the hybrids — 

in Hybridisation 87 

each of the two original species would occur only once. It 
is therefore hardly possible that these should appear at all 
among a small number of experimental plants ; with some 
probability, however, we might reckon upon the appearance 
in the series of a few forms which approach them. 

We meet with an essential difference in those hybrids 
which remain constant in their progeny and propagate 
themselves as truly as the pure species. According to 
Gartner, to this class belong the remarlzably fertile hybrids 
Aquilegia atropmyurea canadensis, Lavatera pseudolbia 
thuringiaca, Geum urhano-rivale, and some Dianthus 
hybrids ; and, according to Wichura, the hybrids of the 
Willow species. For the history of the evolution of plants 
this circumstance is of special importance, since constant 
hybrids acquire the status of new species. The correctness 
of this is evidenced by most excellent observers, and cannot 
be doubted. Gartner had opportunity to follow up Dianthus 
Armeria deltoides to the tenth generation, since it regularly 
propagated itself in the garden. 

With Pisum it was shown by experiment that the 
hybrids form egg and pollen cells of different kinds, and that 
herein lies the reason of the variability of their offspring. 
In other hybrids, likewise, whose offspring behave similarly 
we may assume a like cause ; for those, on the other hand, 
which remain constant the assumption appears justifiable 
that their fertilising cells are all alike and agree with the 
foundation-cell [fertilised ovum] of the hybrid. In the 
opinion of renowned physiologists, for the purpose of 
propagation one pollen cell and one ^gg cell unite in 
Phanerogams* into a single cell, which is capable by 

* In Pisum it is placed beyond doubt that for the formation of the 
new embryo a perfect union of the elements of both fertilising cells 
must take place. How could we otherwise explain that among the 

88 MendeVs Experiments 

assimilation and formation of new cells to become an 
independent organism. This development follows a con- 
stant law, which is founded on the material composition 
and arrangement of the elements which meet in the cell 
in a vivifying union. If the reproductive cells be of the 
same kind and agree with the foundation cell [fertilised 
ovum] of the mother plant, then the development of the 
new individual will follow the same law which rules the 
mother plant. If it chance that an Qgg cell unites with a 
dissimilar pollen cell, we must then assume that between 
those elements of both cells, which determine the mutual 
differences, some sort of compromise is effected. The 
resulting compound cell becomes the foundation of the 
hybrid organism, the development of which necessarily 
follows a different scheme from that obtaining in each of the 
two original species. If the compromise be taken to be a 
complete one, in the sense, namely, that the hybrid embryo 
is formed from cells of like kind, in which the differences 
are entirely and 'permanently accommodated together, the 
further result follows that the hybrids, like any other stable 
plant species, remain true to themselves in their offspring. 
The reproductive cells which are formed in their seed 

offspring of the hybrids both original types reappear in equal numbers 
and with all their peculiarities ? If the influence of the egg cell upon 
the pollen cell were only external, if it fulfilled the role of a nurse 
only, then the result of each artificial fertilisation could be no other 
than that the developed hybrid should exactly resemble the pollen 
parent, or at any rate do so very closely. This the experiments so far 
have in no wise confirmed. An evident proof of the complete union 
of the contents of both cells is afforded by the experience gained on 
all sides that it is immaterial, as regards the form of the hybrid, 
which of the original species is the seed parent or which the pollen 

in Hybridisation 89 

vessels and anthers are of one kind, and agree with the 
fundamental compound cell [fertilised ovum]. 

With regard to those hybrids whose progeny is 'variable 
we may perhaps assume that between the differentiating 
elements of the Q^g and pollen cells there also occurs a 
compromise, in so far that the formation of a cell as 
foundation of the hybrid becomes possible; but, never- 
theless, the arrangement between the conflicting elements 
is only temporary and does not endure throughout the life 
of the hybrid plant. Since in the habit of the plant no 
changes are perceptible during the whole period of vege- 
tation, we must further assume that it is only possible for 
the differentiating elements to liberate themselves from the 
enforced union when the fertilising cells are developed. In 
the formation of these cells all existing elements participate 
in an entirely free and equal arrangement, in which it 
is only the differentiating ones which mutually separate 
themselves. In this way the production would be rendered 
possible of as many sorts of egg and pollen cells as there 
are combinations possible of the formative elements. 

The attribution attempted here of the essential difference 
in the development of hybrids to a permanent or temporary 
union of the differing cell elements can, of course, only 
claim the value of an hypothesis for which the lack of 
definite data offers a wide field. Some justification of the 
opinion expressed lies in the evidence afforded by Pisum 
that the behaviour of each pair of differentiating characters 
in hybrid union is independent of the other differences 
between the two original plants, and, further, that the 
hybrid produces just so many kinds of egg and pollen 
cells as there are possible constant combination forms. 
The differentiating characters of two plants can finally, 
however, only depend upon differences in the composition 

90 MendeVs Experiments 

and grouping of the elements which exist in the foundation- 
cells [fertilised ova] of the same in vital interaction*. 

Even the validity of the law formulated for Pisum 
requires still to be confirmed, and a repetition of the more 
important experiments is consequently much to be desired, 
that, for instance, relating to the composition of the hybrid 
fertilising cells. A differential [element] may easily escape 
the single observer!, which although at the outset may 
appear to be unimportant, may yet accumulate to such 
an extent that it must not be ignored in the total result. 
Whether the variable hybrids of other plant species observe 
an entire agreement must also be first decided experiment- 
ally. In the meantime we may assume that in material 
points a difference in principle can scarcely occur, since the 
unity in the developmental plan of organic life is beyond 

In conclusion, the experiments carried out by Kolreuter, 
Gartner, and others with respect to the transformation of 
one species into another hy artificial fertilisation merit 
special mention. A special importance has been attached 
to these experiments, and Gartner reckons them among 
"the most difficult of all in hybridisation." 

If a species A is to be transformed into a species B, 
both must be united by fertilisation and the resulting 
hybrids then be fertilised with the pollen of B ; then, out 
of the various offspring resulting, that form would be 
selected which stood in nearest relation to B and once 
more be fertilised with B pollen, and so continuously until 
finally a form is arrived at which is like B and constant in 

* ^^ Welche in den Grundzellen derselben in lebendiger Wechsel- 
ivirkung stehen.^' 

+ " Dem einzelnen Beobachter kann leicTit ein Differenziale ent- 

i7i Hybridisation 91 

its progeny. By this process the species A would change 
into the species B. Gartner alone has effected thirty such 
experiments vnth. plants of genera Aquilegia, Dlanthus, 
Geum, Lavatera, Lychnis, Malva, Nicotiana, and QEnothera. 
The period of transformation was not alike for all species. 
"While with some a triple fertilisation sufficed, with others 
this had to be repeated five or six times, and even in the 
same species fluctuations were observed in various experi- 
ments. Gartner ascribes this difference to the circumstance 
that "the specific \typische\ force by which a species, during 
reproduction, effects the change and transformation of the 
maternal type varies considerably in different plants, and 
that, consequently, the periods mthin which the one species 
is changed into the other must also vary, as also the number 
of generations, so that the transformation in some species 
is perfected in more, and in others in fewer generations." 
Further, the same observer remarks "that in these trans- 
formation experiments a good deal depends upon which type 
and which individual be chosen for further transformation." 
If it ma)^ be assumed that in these experiments the 
constitution of the forms resulted in a similar way to that 
of Pisum, the entire process of transformation would find 
a fairly simple explanation. The hybrid forms as many 
kinds of egg cells as there are constant combinations 
possible of the characters conjoined therein, and one of 
these is always of the same kind as the fertilising pollen 
cells. Consequently there always exists the possibility with 
all such experiments that even from the second fertilisation 
there may result a constant form identical with that of the 
pollen parent. Whether this really be obtained depends in 
each separate case upon the number of the experimental 
plants, as well as upon the number of differentiating 
characters which are united by the fertilisation. Let us, 

92 MeiideVs Experiments 

for instance, assume that the plants selected for experiment 
differed in three characters, and the species ABC i^ to 
be transformed into the other species ahc by repeated 
fertilisation with the pollen of the latter ; the hybrids 
resulting from the first cross form eight different kinds of 
Qgg cells, viz. : 

ABC J A Be, AbC, aBC, Abe, aBc, abC, abc. 

These in the second year of experiment are united again 
with the pollen cells abc, and we obtain the series 

AaBbCc + AaBbe + AabCe + aBbCe 

+ A abc + aBbe + abCe + abc. 

Since the form abc occurs once in the series of eight 
components, it is consequently little likely that it would be 
missing among the experimental plants, even were these 
raised in a smaller number, and the transformation would 
be perfected already by a second fertilisation. If by chance 
it did not appear, then the fertilisation must be repeated 
with one of those forms nearest akin, Aabc, aBbc, abCc. 
It is perceived that such an experiment must extend the 
farther the smaller the number of experimental 'plants and 
the larger the number of differentiating characters in the 
two original species; and that, furthermore, in the same 
species there can easily occur a delay of one or even of two 
generations such as Gartner observed. The transformation 
of widely divergent species could generally only be completed 
in five or six years of experiment, since the number of 
different Qgg cells which are formed in the hybrid increases 
in square ratio with the number of differentiating characters. 

Gartner found by repeated experiments that the respec- 
tive period of transformation varies in many species, so that 
frequently a species A can be transformed into a species B 

in Hybridisation 93 

a generation sooner than can species B into species A . He 
deduces therefrom that Kolreuter's opinion can hardly be 
maintained that " the two natures in hybrids are perfectly 
in equilibrium." It appears, however, that Kolreuter does 
not merit this criticism, but that Gartner rather has over- 
looked a material point, to which he himself elsewhere 
draws attention, viz. that " it depends which individual is 
chosen for further transformation." Experiments which in 
this connection were carried out with two species of Pisum 
demonstrated that as regards the choice of the fittest 
individuals for the purpose of further fertilisation it may 
make a great difference which of two species is transformed 
into the other. The two experimental plants differed in 
five characters, while at the same time those of species A 
were all dominant and those of species B all recessive. 
For mutual transformation A was fertilised with pollen of 
B^ and B with pollen of A, and this was repeated with 
both hybrids the following year. With the first experiment 

-J there were eighty-seven plants available in the third 

year of experiment for the selections of individuals for 
further crossing, and these were of the possible thirty-two 


forms ; with the second experiment -j: seventy-three plants 

resulted, which agreed throughout perfectly in habit with 
the pollen parent; in their internal composition, however, 
they must have been just as varied as the forms of the 
other experiment. A definite selection was consequently 
only possible with the first experiment ; with the second 
some plants selected at random had to be excluded. Of 
the latter only a portion of the flowers were crossed with 
the A pollen, the others were left to fertilise themselves. 
Among each five plants which were selected in both 

94 Mendel's Experiments 

experiments for fertilisation there agreed, as the following 
year's culture showed, with the pollen parent: — 

1st Experiment. 

2nd Experiment. 

2 plants 


in all characters 

3 „ 


)) '^ 5J 


2 plants 

?j 3 „ 


2 „ 

u 2 „ 


1 plant 

,, 1 character 

In the first experiment, therefore, the transformation 
was completed; in the second, which was not continued 
further, two more fertilisations would probably have been 

Although the case may not frequently occur that the 
dominant characters belong exclusively to one or the other 
of the original parent plants, it will always make a difference 
which of the two possesses the majority. If the pollen parent 
shows the majority, then the selection of forms for further 
crossing will afford a less degree of security than in the 
reverse case, which must imply a delay in the period of 
transformation, provided that the experiment is only 
considered as completed when a form is arrived at which 
not only exactly resembles the pollen plant in form, but 
also remains as constant in its progeny. 

Gartner, by the results of these transformation experi- 
ments, was led to oppose the opinion of those naturalists 
who dispute the stability of plant species and believe in a 
continuous evolution of vegetation. He perceives in the 
complete transformation of one species into another an 
indubitable proof that species are fixed within limits 
beyond which they cannot change. Although this opinion 
cannot be unconditionally accepted we find on the other 
hand in Gartner's experiments a noteworthy confirmation 

in Hyhridisatio7i 95 

of that supposition regarding variability of cultivated 
plants which has already been expressed. 

Among the experimental species there were cultivated 
plants, such as Aquilegia atropurpurea and canadensis, 
Dianthus caryophyllus, chinensis, and japonicus, Nicotiana 
rustica and paniculata, and hybrids between these species 
lost none of their stability after four or five generations*. 

* [The argument of these two last paragraphs appears to be that 
though the general mutability of natural species might be doubtful, 
yet among cultivated plants the transference of characters may be 
accomplished, and may occur by integral steps until one species is 
definitely "transformed" into the other.] 


By G. Mendel. 

{Communicated to the Meeting 9 June, 1869*.) 

Although I have already undertaken many experiments 
in fertilisation between species of Hieracium, I have only 
succeeded in obtaining the following 6 hybrids, and only 
from one to three specimens of them. 

H. Auricula 2 x H. aurantiacum c? 

H. Auricula ^ ^ H. Pilosella $ 

H. Auricula ^ x H. pratense S 

H. eckioidesf ^ x H. aurantiacum S 

H. prcealtum ^ y- H. flagellare Rchb. $ 

H. prwaltum ^ ^ H. aurantiacum $ 

The difficulty of obtaining a larger number of hybrids 
is due to the minuteness of the flowers and their peculiar 
structure. On account of this circumstance it was seldom 
possible to remove the anthers from the flowers chosen for 

* [Published in Verh. naturf. Ver. Brilnn, Abhandlungen, viii. 1869, 
p. 26, which appeared in 1870.] 

t The plant used in this experiment is not exactly the typical 
H. echioides. It appears to belong to the series transitional to 
H. prcealtum, but approaches more nearly to H. echioides and for 
this reason was reckoned as belonging to the latter. 

MendeVs Experiments with Hieracimn 97 

fertilisation without either letting pollen get on to the 
stigma or injuring the pistil so that it withered away. 
As is well kno^vn, the anthers are united to form a tube, 
which closely embraces the pistil. As soon as the flower 
opens, the stigma, already covered with pollen, protrudes. 
In order to prevent self-fertilisation the anther-tube must 
be taken out before the flower opens, and for this purpose 
the bud must be slit up with a fine needle. If this 
operation is attempted at a time when the pollen is mature, 
which is the case two or three days before the flower opens, 
it is seldom possible to prevent self-fertilisation: for with 
every care it is not easily possible to prevent a few pollen 
gTains getting scattered and communicated to the stigma. 
No better result has been obtained hitherto by removing 
the anthers at an earlier stage of development. Before the 
approach of maturity the tender pistil and stigma are ex- 
ceedingly sensitive to injury, and even if they are not actually 
injured, they generally wither and dry up after a little 
time if deprived of their protecting investments. I hope 
to obviate this last misfortune by placing the plants after 
the operation for two or three days in the damp atmosphere 
of a greenhouse. An experiment lately made with H. 
Auricula treated in this way gave a good result. 

To indicate the object with which these fertilisation 
experiments were undertaken, I venture to make some 
preliminary remarks respecting the genus Hieracium. This 
genus possesses such an extraordinary profusion of distinct 
forms that no other genus of plants can compare with it. 
Some of these forms are distinguished by special peculiarities 
and may be taken as type-forms of species, while all the 
rest represent intermediate and transitional forms by which 
the type-forms are connected together. The difficulty in 
the separation and delimitation of these forms has demanded 

B. 7 

98 MendeVs Experiments 

the close attention of the experts. Regarding no other 
genus has so much been written or have so many and such 
fierce controversies arisen, without as yet coming to a 
definite conclusion. It is obvious that no general under- 
standing can be arrived at, so long as the value and 
significance of the intermediate and transitional forms is 

Regarding the question whether and to what extent 
hybridisation plays a part in the production of this wealth 
of forms, we find very various and conflicting views held 
by leading botanists. While some of them maintain that 
this phenomenon has a far-reaching influence, others, for 
example. Fries, will have nothing to do with hybrids in 
Hieracia. Others take up an intermediate position; and 
while granting that hybrids are not rarely formed between 
the species in a wild state, still maintain that no great 
importance is to be attached to the fact, on the ground 
that they are only of short duration. The [suggested] 
causes of this are partly their restricted fertility or complete 
sterility ; partly also the knowledge, obtained by experiment, 
that in hybrids self-fertilisation is always prevented if 
pollen of one of the parent-forms reaches the stigma. On 
these grounds it is regarded as inconceivable that Hieracium 
hybrids can constitute and maintain themselves as fully 
fertile and constant forms when growing near their pro- 

The question of the origin of the numerous and constant 
intermediate forms has recently acquired no small interest 
since a famous Hieracium specialist has, in the spirit of 
the Darwinian teaching, defended the view that these 
forms are to be regarded as [arising] from the trans- 
mutation of lost or still existing species. 

From the nature of the subject it is clear that without 

with Hieracium 99 

an exact knowledge of the structure and fertility of the 
hybrids and the condition of their offspring through several 
generations no one can undertake to determine the possible 
influence exercised by hybridisation over the multiplicity 
of intermediate forms in Hieracium. The condition of 
the Hieracium hybrids in the range we are concerned with 
must necessarily be determined by experiments ; for we do 
not possess a complete theory of hybridisation, and we may 
be led into erroneous conclusions if we take rules deduced 
from observation of certain other hybrids to be Laws of 
hybridisation, and try to apply them to Hieracium without 
further consideration. If by the experimental method we 
can obtain a sufficient insight into the phenomenon of 
hybridisation in Hieracium, then by the help of the ex- 
perience which has been collected respecting the structural 
relations of the wild forms, a satisfactory judgment in 
regard to this question may become possible. 

Thus we may express the object which was sought after 
in these experiments. I venture now to relate the very 
slight results which I have as yet obtained with reference 
to this object. 

1. Respecting the structure of the hybrids, we have 
to record the striking phenomenon that the forms hitherto 
obtained by similar fertilisation are not identical. The 
hybrids H. pra^altum 9 x H. aurantiacum $ and H. Auri- 
cula 2 ^ H. aurantiacum S are each represented by two, 
and H. Auricula 2 ^ H pratense 6 by three individuals, 
while as to the remainder only one of each has been 

If we compare the individual characters of the hybrids 
with the corresponding characters of the two parent types, 
we find that they sometimes present intermediate structures, 


100 MendeFs Experiments 

but are sometimes so near to one of the parent characters 
that the [corresponding] character of the other has receded 
considerably or almost evades observation. So, for instance, 
we see in one of the two forms of H. Auricula ? x ZT. 
aurantiacum 6 pure yellow disc-florets ; only the petals 
of the marginal florets are on the outside tinged with red 
to a scarcely noticeable degree : in the other on the contrary 
the colour of these florets comes very near to H. aurantiacum, 
only in the centre of the disc the orange red passes into a 
deep golden-yellow. This diff'erence is noteworthy, for the 
flower-colour in Hieracium has the value of a constant 
character. Other similar cases are to be found in the 
leaves, the peduncles, &c. 

If the hybrids are compared with the parent types as 
regards the sum total of their characters, then the two 
forms of H. prwaltum ^ ^ H. aurantiacum S constitute 
approximately intermediate forms which do not agree in 
certain characters. On the contrary in H. Auricula ^ x ff. 
aurantiacum c? and in If. Auricula '^ x H. pratense S we 
see the forms widely divergent, so that one of them is 
nearer to the one and the other to the other parental type, 
while in the case of the last-named hybrid there is still a 
third which is almost precisely intermediate between them. 

The conviction is then forced on us that we have here 
only single terms in an unknown series which may be 
formed by the direct action of the pollen of one species on 
the egg-cells of another. 

2. With a single exception the hybrids in question 
form seeds capable of germination. H. echioides ^ x H. 
aurantiacum 6 may be described as fully fertile ; *H. prw- 
altum ^ X H. flagellare $ as fertile ; H. prwaltum ^ x H. 
aurantiacum $ and H. Auricula ^ x H. pratense 6 as 

with Hieraciiim 101 

partially fertile ; H. Auricula ^ x ff. Pilosella $ as slightly 
fertile, and H. Auricula ^ ^H. aurantiacum $ as unfertile. 
Of the two forms of the last named hybrid, the red-flowered 
one was completely sterile, but from the yellow-flowered 
one a single well-formed seed was obtained. Moreover it 
must not pass unmentioned that among the seedlings of the 
partially fertile hybrid H. pra?altum ^ ^ IT. aurantiacum ^ 
there was one plant which possessed full fertility. 

[3.] As yet the ofl'spring produced by self-fertilisation 
of the hybrids have not varied, but agree in their characters 
both with each other and with the hybrid plant from which 
they were derived. 

From H. proealtum 2 y- H. flagellare S two generations 
have flowered ; from H. echioides 2 ■>< H. aurantiacum 6 , 
H. prwaltum '^ ^ IT. aurantiacum 6 , H. Auricula ^ x ff. 
Pilosella S one generation in each case has flowered. 

4. The fact must be declared that in the case of the 
fully fertile hybrid H. echioides ^ x H. aurantiacum $ the 
pollen of the parent types was not able to prevent self- 
fertilisation, though it was applied in great quantity to the 
stigmas protruding through the anther-tubes when the 
flowers opened. 

From two flower-heads treated in this way seedlings 
were produced resembling this hybrid plant. A very 
similar experiment, carried out this summer with the 
partially fertile H. prwaltum ^ x H. aurantiacum 6 led to 
the conclusion that those flower-heads in which pollen 
of the parent type or of some other species had been 
applied to the stigmas, developed a notably larger number 
of seeds than those which had been left to self-fertilisation 
alone. The explanation of this result must only be sought 
in the circumstance that as a large part of the pollen -grains 

102 Mender s Experiments 

of the hybrid, examined microscopically, show a defective 
structure, a number of egg-cells capable of fertilisation do 
not become fertilised by their own pollen in the ordinary 
course of self-fertilisation. 

It not rarely happens that in fully fertile species in the 
wdld state the formation of the pollen fails, and in many 
anthers not a single good grain is developed. If in these 
cases seeds are nevertheless formed, such fertilisation must 
have been effected by foreign pollen. In this way hybrids 
may easily arise by reason of the fact that many forms 
of insects, notably the industrial Hymenoptera, visit the 
flowers of Hieracia with great zeal and are responsible for 
the pollen which easily sticks to their hairy bodies reaching 
the stigmas of neighbouring plants. 

From the few facts that I am able to contribute it 
will be evident the work scarcely extends beyond its first 
inception. I must express some scruple in describing in 
this place an account of experiments just begun. But the 
conviction that the prosecution of the proposed experiments 
will demand a whole series of years, and the uncertainty 
whether it will be granted to me to bring the same to a 
conclusion have determined me to make the present 
communication. By the kindness of Dr Nageli, the 
Munich Director, who was good enough to send me species 
which were wanting, especially from the Alps, I am in a 
position to include a larger number of forms in my 
experiments. I venture to hope even next year to be able 
to contribute something more by way of extension and con- 
firmation of the present account. 

If finally we compare the described result, still very 
uncertain, with those obtained by crosses made between 
forms of Pisum, which I had the honour of communi- 
cating in the year 1865, we find a very real distinction. 

with Hieradmn 103 

In Pisum the hybrids, obtained from the immediate 
crossing of two forms, have in all cases the same type, 
but their posterity, on the contrary, are variable and 
follow a definite law in their variations. In Hieracium 
according to the present experiments the exactly opposite 
phenomenon seems to be exhibited. Already in describing 
the Pisum experiments it was remarked that there are 
also hybrids whose posterity do not vary, and that, for 
example, according to Wichura the hybrids of Salix 
reproduce themselves like pure species. In Hieracium 
we may take it we have a similar case. Whether from 
this circumstance we may venture to draw the conclusion 
that the polymorphism of the genera Salix and Hieracium 
is connected with the special condition of their hybrids is 
still an open question, which may well be raised but not 
as yet answered. 


" The most fertile men of science have made blunders, and their 
consciousness of such slips has been retribution enough; it is 
only their more sterile critics who delight to dwell too often 
and too long on such mistakes?^ Biometrika, 1901. 


On the rediscovery and confirmation of Mendel's Law by 
de Vries, Correns, and Tschermak two years ago, it became 
clear to many naturalists, as it certainly is to me, that we 
had found a principle which is destined to play a part in 
the Study of Evolution comparable only with the achieve- 
ment of Darwin — that after the weary halt of forty years 
we have at last begun to march. 

If we look back on the post-Darwinian period we 
recognize one notable effort to advance. This effort — 
fruitful as it proved, memorable as it must ever be — was 
that made by Galton when he enuntiated his Law of 
Ancestral Heredity, subsequently modified and restated 
by Karl Pearson. Formulated after long and laborious 
inquiry, this principle beyond question gives us an 
expression including and denoting many phenomena in 
which previously no regularity had been detected. But 

A Defence of Mendel's Prhiciples of Heredity 105 

to practical naturalists it was evident from the first that 
there are great groups of facts which could not on any 
interpretation be brought within the scope of Galton's 
Law, and that by no emendation could that Law be 
extended to reach them. The existence of these phen- 
omena pointed to a different physiological conception of 
heredity. Now it is precisely this conception that Mendel's 
Law enables us to form. Whether the Mendelian principle 
can be extended so as to include some apparently Galtonian 
cases is another question, respecting which we have as yet 
no facts to guide us, but we have certainly no warrant for 
declaring such an extension to be impossible. 

Whatever answer the future may give to that question, 
it is clear from this moment that every case which obeys 
the Mendelian principle is removed finally and irretrievably 
from the operations of the Law of Ancestral Heredity. 

At this juncture Professor Weldon intervenes as a 
professed exponent of Mendel's work. It is not perhaps 
to a devoted partisan of the Law of Ancestral Heredity 
that we should look for the most appreciative exposition of 
Mendel, but some bare measure of care and accuracy in 
representation is demanded no less in justice to fine work, 
than by the gravity of the issue. 

Professor Weldon's article appears in the current number 
of Biometrika, Vol. L Pt. ii. which reached me on Saturday, 
Feb. 8. The paper opens with what purports to be a 
restatement of Mendel's experiments and results. In this 
"restatement" a large part of Mendel's experiments — 
perhaps the most significant — are not referred to at all. 
The perfect simplicity and precision of Mendel's own 
account are destroyed ; with the result that the reader of 
Professor Weldon's paper, unfamiliar with Mendel's own 
memoir, can scarcely be blamed if he fail to learn the 

106 A Defence of MendeVs 

essence of the discovery. Of Mendel's conception of the 
hybrid as a distinct entity with characters proper to itself, 
apart from inheritance — the most novel thing in the 
whole paper — Professor Weldon gives no word. Upon this 
is poured an undigested mass of miscellaneous "facts" 
and statements from which the reader is asked to conclude, 
first, that a proposition attributed to Mendel regarding 
dominance of one character is not of "general""^ application, 
and finally that "all work based on Mendel's method" is 
"vitiated" by a "fundamental mistake," namely "the 
neglect of ancestry!." 

To find a parallel for such treatment of a great theme 
in biology we must go back to those writings of the orthodox 
which followed the appearance of the " Origin of Species." 

On 17th December 1900 I delivered a Report to the 
Evolution Committee of the Royal Society on the experi- 
ments in Heredity undertaken by Miss E. R. Saunders and 
myself. This report has been offered to the Society for 
publication and will I understand shortly appear. In it we 
have attempted to show the extraordinary significance of 
Mendel's principle, to point out what in his results is 
essential and what subordinate, the ways in which the 
principle can be extended to apply to a diversity of more 
complex phenomena — of which some are incautiously cited 

* The words "general" and ''universal" appear to be used by 
Professor Weldon as interchangeable. Cp. Weldon, p. 235 and 
elsewhere, with Abstract given below. 

t These words occur p. 252 : "The fundamental mistake which 
vitiates all work based upon Mendel's method is the neglect of 
ancestry, and the attempt to regard the whole effect upon offspring pro- 
duced by a particular parent, as due to the existence in the parent of 
particular structural characters, &c." As a matter of fact the view 
indicated in these last words is especially repugnant to the Mendelian 
principle, as will be seen. 

PrincijjUs of Heredity 107 

by Professor Weldon as conflicting facts — and lastly to 
suggest a few simple terms without wliich (or some equi- 
valents) the discussion of such phenomena is difficult. 
Though it is impossible here to give an outline of facts and 
reasoning there set out at length, I feel that his article 
needs an immediate reply. Professor Weldon is credited 
with exceptional familiarity with these topics, and his paper 
is likely to be accepted as a sufficient statement of the case. 
Its value will only be known to those who have either 
worked in these fields themselves or have been at the 
trouble of thoughtfully studying the original materials. 

The nature of Professor Weldon's article may be most 
readily indicated if I quote the summary of it issued in a 
paper of abstracts sent out with Review copies of the Part. 
This paper was most courteously sent to me by an editor 
of Biometriha in order to call my attention to the article 
on Mendel, a subject in which he knew me to be interested. 
The abstract is as follows. 

"Few subjects have excited so much interest in the last 
year or two as the laws of inheritance in hybrids. Professor 
W. F. R. Weldon describes the results obtained by Mendel by 
crossing races of Peas which dififered in one or more of seven 
characters. From a study of the work of other observers, and 
from examination of the 'Telephone' group of hybrids, the 
conclusion is drawn that Mendel's results do not justify any 
general statement concerning inheritance in cross-bred Peas. A 
few striking cases of other cross-bred plants and animals are 
quoted to show that the results of crossing cannot, as Mendel 
and his followers suggest, be predicted from a knowledge of the 
characters of the two parents crossed without knowledge of the 
more remote ancestry." 

Such is the judgment a fellow-student passes on this 

" Voyaging through strange seas of thought aloneJ^ 

108 A Defence of MendeVs 

The only conclusion which most readers could draw 
from this abstract and indeed from the article it epitom- 
izes, is that Mendel's discovery so far from being of 
paramount importance, rests on a basis which Professor 
Weldon has shown to be insecure, and that an error has 
come in through disregard of the law of Ancestral Heredity. 
On examining the paper it is perfectly true that Professor 
Weldon is careful nowhere directl}^ to question Mendel's 
facts or his interpretation of them, for Avhich indeed in 
some places he even expresses a mild enthusiasm, but there 
is no mistaking the general purpose of the paper. It must 
inevitably produce the impression that the importance of 
the work has been greatly exaggerated and that supporters 
of current views on Ancestry may reassure themselves. 
That this is Professor Weldon's own conclusion in the 
matter is obvious. After close study of his article it is 
evident to me that Professor Weldon's criticism is baseless 
and for the most part irrelevant, and I am strong in the 
conviction that the cause which will sustain damage from 
this debate is not that of Mendel 

I. The Mendelian Principle of Purity of (jerm- Cells 
AND THE Laws of Heredity based on Ancestry. 

Professor Weldon's article is entitled "Mendel's Laws 
of Alternative Inheritance in Peas." This title expresses 
the scope of Mendel's work and discovery none too 
precisely and even exposes him to distinct miscon- 

To begin with, it says both too little and too much. 
Mendel did certainly determine Laws of Inheritance in 

Princijyles of Heredity 109 

peas — not precisely the laws Professor Weldon has been 
at the pains of drafting, but of that anon. Having done 
so, he knew what his discovery was worth. He saw, and 
rightly, that he had found a principle which must govern 
a wide area of phenomena. He entitles liis paper therefore 
*' Versuchs ilber Pflanzen- Hybrid en, ^^ or, Experiments in 

Nor did Mendel start at first with any particular 
intention respecting Peas. He tells us himself that he 
wanted to find the laws of inheritance in hybrids, which 
he suspected were definite, and that after casting about 
for a suitable subject, he found one in peas, for the reasons 
he sets out. 

In another respect the question of title is much more 
important. By the introduction of the word "Alternative " 
the suggestion is made that the Mendelian principle applies 
peculiarly to cases of " alternative " inheritance. Mendel 
himself makes no such limitation in his earlier paper, 
though perhaps by rather remote implication in the second, 
to which the reader should have been referred. On the 
contrary, he wisely abstains from prejudicial consideration 
of unexplored phenomena. 

To understand the significance of the word "alternative" 
as introduced by Professor Weldon we must go back a 
little in the history of these studies. In the year 1897 
Galton formally announced the Law of Ancestral Heredity 
referred to in the Introduction, having previously "stated 
it briefly and with hesitation" in Natural Inheritance, 
p. 134. In 1898 Professor Pearson published his modifi- 
cation and generalisation of Galton's Law, introducing a 
correction of admitted theoretical importance, though it is 
not in question that the principle thus restated is funda- 

110 A Defence of Mendel's 

mentally not very different from Galton's*. It if^ an 
essential part of the Galton-Pearson Law of Ancestral 
Heredity that in calculating the probable structure of each 
descendant the structure of each several ancestor must be 
brought to account. 

Professor Weldon now tells us that these two papers 
of Galton and of Professor Pearson have "given us an 
expression for the effects of blended inheritance which 
seems likely to prove generally applicable, though the 
constants of the equations which express the relation 
between divergence from the mean in one generation, and 
that in another, may require modification in special cases. 
Our knowledge of particulate or mosaic inheritance, and of 
alternatim inheritance, is however still rudimentary, and 
there is so much contradiction between the results obtained 
by different observers, that the evidence available is difficult 
to appreciate." 

But Galton stated (p. 401) in 1897 that his statistical 
law of heredity "appears to be universally applicable to 
bi-sexual descent." Pearson in re-formulating the principle 
in 1898 made no reservation in regard to "alternative" 
inheritance. On the contrary he writes (p. 393) that "if 
Mr Galton's law can be firmly established, it is a complete 
solution, at any rate to a first approximation, of the whole 
problem of heredity" and again (p. 412) that "it is highly 
probable that it [this law] is the simple descriptive state- 

* I greatly regret that I have not a precise understanding of the 
basis of the modification proposed by Pearson. His treatment is in 
algebraical form and beyond me. Nevertheless I have every confidence 
that the arguments are good and the conclusion sound. I trust it 
may not be impossible for him to provide the non-mathematical reader 
with a paraphrase of his memoir. The arithmetical differences between 
the original and the modified law are of course clear. 

Princij^les of Heredity 111 

ment which brings into a single focus all the complex 
lines of hereditary influence. If Darwinian evolution be 
natural selection combined with heredity, then the single 
statement which embraces the whole field of heredity must 
prove almost as epoch-making as the law of gravitation 
to the astronomer*." 

As I read there comes into my mind that other fine 
passage where Professor Pearson warns us 

"There is an insatiable desire in the human breast 
"to resume in some short formula, some brief 
"statement, the facts of human experience. It leads 
" the savage to ' account ' for all natural phenomena 
"by deifying the wind and the stream and the tree. 
" It leads civilized man, on the other hand, to express 
"his emotional experience in works of art, and his 
"physical and mental experience in the formulae or 
"so-called laws of science f." 

No naturalist who had read Galton's paper and had 
tried to apply it to the facts he knew could fail to see 
that here was a definite advance. We could all perceive 
phenomena that were in accord with it and there was no 
reasonable doubt that closer study would prove that accord 
to be close. It was indeed an occasion for enthusiasm, 
though no one acquainted with the facts of experimental 
breeding could consider the suggestion of universal applica- 
tion for an instant. 

* I have searched Professor Pearson's paper in vain for any con- 
siderable reservation regarding or modification of this general state- 
ment. Professor Pearson enuntiates the law as " only correct on 
certain limiting hypotheses," but he declares that of these the most 
important is " the absence of reproductive selection, i.e. the negligible 
correlation of fertility with the inherited character, and the absence 
of sexual selection." The case of in-and-in breeding is also reserved. 

t K. Pearson, Grammar of Science, 2nd ed. 1900, p. 36. 

112 A Defence of Mendel's 

But two years have gone by, and in 1900 Pearson 
writes * that the values obtained from the Law of Ancestral 

" seem to fit the observed facts fairly well in the case of 
^^ blended inheritance. In other words we have a 
"certain amount of evidence in favour of the 
"conclusion : That whenever the sexes are equipotent, 
^''hlend their characters and mate pangamously, all 
^^ characters will he inherited at the same rate^' 

or, again in other words, that the Law of Ancestral Heredity 
after the glorious launch in 1898 has been home for a 
complete refit. The top-hamper is cut down and the vessel 
altogether more manageable ; indeed she looks trimmed 
for most weathers. Each of the qualifications now intro- 
duced wards off whole classes of dangers. Later on (pp. 
487 — 8) Pearson recites a further list of cases regarded as 
exceptional. " All characters will be inherited at the same 
rate " might indeed almost be taken to cover the results in 
Mendelian cases, though the mode by which those results 
are arrived at is of course wholly different. 

Clearly we cannot speak of the Law of Gravitation now. 
Our Tycho Brahe and our Kepler, with the yet more distant 
Newton, are appropriately named as yet to comet. 

But the truth is that even in 1898 such a comparison 
was scarcely happy. Not to mention moderns, these high 
hopes had been finally disposed of by the work of the 
experimental breeders such as Kolreuter, Knight, Herbert, 
Gartner, Wichura, Godron, Naudin, and many more. To 
have treated as non-existent the work of this group of 
naturalists, who alone have attempted to solve the problems 

* Grammar of Science, 2nd ed. 1900, p. 480. 
t Phil. Trans. 1900, vol. 195, A, p. 121. 

Principles of Heredity 113 

of heredity and species — Evolution, as we should now say — 
by the only sound method — experimental hreeding — to 
leave out of consideration almost the whole block of 
evidence collected in Animals and Plants — Darwin's finest 
legacy as I venture to declare — was unfortunate on the 
part of any exponent of Heredity, and in the writings of a 
professed naturalist would have been unpardonable. But 
even as modified in 1900 the Law of Ancestral Heredity 
is heavily over-sparred, and any experimental breeder could 
have increased Pearson's list of unconformable cases by as 
many again. . 

But to return to Professor Weldon. He now repeats 
that the Law of Ancestral Heredity seems likely to prove 
generally applicable to blended inheritance, but that the 
case of alternative inheritance is for the present reserved. 
We should feel more confidence in Professor "Weldon's 
exposition if he had here reminded us that the special 
case which fitted Galton's Law so well that it emboldened 
him to announce that principle as apparently " universally 
applicable to bi-sexual descent" was one of alternative 
inheritance — namely the coat-colour of Basset-hounds. 
Such a fact is, to say the least, ominous. Pearson, in 
speaking (1900) of this famous case of Galton's, says that 
these phenomena of alternative inheritance must be treated 
separately (from those of blended inheritance)^, and for 
them he deduces a proposed ''''law of reversion" based of 
course on ancestry. He writes, "In both cases we may 
speak of a law of ancestral heredity, but the first predicts 
the probable character of the individual produced by a 

* " If this be done, we shall, I venture to think, keep not only our 
minds, but our points for observation, clearer ; and further, the failure 
of Mr Galton's statement in the one case will not in the least afifect 
its validity in the other." Pearson (32), p. 143. 

B. 8 

114 A Defence of Menders 

given ancestry, while the second tells us the percentages 
of the total offspring which on the average revert to each 
ancestral type*." 

With the distinctions between the original Law of 
Ancestral Heredity, the modified form of the same law, 
and the Law of Reversion, important as all these considera- 
tions are, we are not at present concerned. 

For the Mendelian principle of heredity asserts a 
proposition absolutely at variance with all the laws of 
ancestral heredity, however formulated. In those cases to 
which it applies strictly, this principle declares that the 
cross-breeding of parents need not diminish the purity of 
their germ-cells or consequently the purity of their off- 
spring. When in such cases individuals bearing opposite 
characters, A and B, are crossed, the germ-cells of the 
resulting cross-bred, AB, are each to be bearers either 
of character A or of character B, not both. 

Consequently when the cross-breds breed either together 
or with the pure forms, individuals will result of the forms 
A A, AB, BA, BBt Of these the forms A A and BB, 
formed by the union of similar germs, are stated to be as 
pure as if they had had no cross in their pedigree, and 
henceforth their offspring will be no more likely to depart 
from the A type or the B type respectively, than those of 
any other originally pure specimens of these types. 

Consequently in such examples it is 7iof the fact that 
each ancestor must be brought to account as the Galton- 
Pearson Law asserts, and we are clearly dealing with a 
physiological phenomenon not contemplated by that Law 
at all. 

* Grammar of Science, 1900, p. 494. See also Pearson, Proc. Roy. 
Soc. 1900, Lxvi. pp. 142-3. 

t On an average of cases, in equal numbers, as Mendel found. 

Principles of Heredity 115 

Every case therefore which obeys the Mendelian principle 
is in direct contradiction to the proposition to which Pro- 
fessor Weldon's school is committed, and it is natural that 
he should be disposed to consider the Mendelian principle 
as applying especially to "alternative" inheritance, while 
the law of Galton and Pearson is to include the phenomenon 
of blended inheritance. The latter, he tells us, is "the 
most usual case," a view which, if supported by evidence, 
might not be without value. 

It is difficult to blame those who on first acquaintance 
concluded Mendel's principle can have no strict application 
save to alternative inheritance. Whatever blame there is 
in this I share with Professor Weldon and those whom he 
follows. Mendel's own cases were almost all alternative ; 
also the fact of dominance is very dazzling at first. But 
that was two years ago, and when one begins to see clearly 
again, it does not look so certain that the real essence of 
Mendel's discovery, the purity of germ-cells in respect of 
certain characters, may not apply also to some phenomena 
of blended inheritance. The analysis of this possibility 
would take us to too great length, but I commend to those 
who are more familiar with statistical method, the consider- 
tion of this question : whether dominance being absent, 
indefinite, or suppressed, the phenomena of heritages 
completely blended in the zygote, may not be produced 
by gametes presenting Mendelian purity of characters. 
A brief discussion of this possibility is given in the 
Introduction, p. 31. 

Very careful inquiry would be needed before such a 
possibility could be negatived. For example, we know 
that the Laws based on Ancestry can apply to alternative 
inheritance ; witness the case of the Basset-hounds. Here 
there is no simple Mendelian dominance ; but are we sure 


116 A Defence of MendeVs 

there is no purity of germ-cells ? The new conception goes 
a long way and it may well reach to such facts as these. 

But for the present we will assume that Mendel's 
principle applies only to certain phenomena of alternative 
inheritance, which is as far as our warrant yet runs. 

No close student of the recent history of evolutionary 
thought needs to be told what the attitude of Professor 
Weldon and his followers has been tow^ards these same 
disquieting and unwelcome phenomena of alternative 
inheritance and discontinuity in variation. Holding at 
first each such fact for suspect, then treating them as rare 
and negligible occurrences, he and his followers have of 
late come slowly to accede to the facts of discontinuity a 
bare and grudging recognition in their scheme of evolution*. 

Therefore on the announcement of that discovery which 
once and for all ratifies and consolidates the conception of 
discontinuous variation, and goes far to define that of 
alternative inheritance, giving a finite body to what before 
was vague and tentative, it is small wonder if Professor 
Weldon is disposed to criticism rather than to cordiality. 

We have now seen what is the essence of Mendel's 
discovery based on a series of experiments of unequalled 
simplicity which Professor Weldon does not venture to 

* Eead in this connexion Pearson, K., Grammar of Science, 2nd 
ed. 1900, pp. 390—2. 

Professor Weldon even now opens his essay with the statement — 
or perhaps reminiscence — that *' it is perfectly possible and indeed 
probable that the difference between these forms of inheritance 
[blended, mosaic, and alternative] is only one of degree." This may be 
true ; but reasoning favourable to this proposition could equally be 
used to prove the difference between mechanical mixture and chemical 
combination to be a difference of degree. 

Principles of Heredity 117 

11. Mendel and the Critic's Version of him. 
The ^^ Law of Dominance." 

I proceed to the question of dominance which Professor 
Weldon treats as a prime issue, almost to the virtual con- 
cealment of the great fact of gametic purity. 

Cross-breds in general, AB and BA, named above, 
may present many appearances. They may all be indis- 
tiuguishable from A, or from B ; some may appear J.'s 
and some ^'s ; they may be patchworks of both ; they may 
be blends presenting one or many grades between the two ; 
and lastly they may have an appearance special to themselves 
(heing in the latter case^ as it oft en happens^ ^Reversionary"), 
a possibility which Professor Weldon does not stop to 
consider, though it is the clue that may unravel many 
of the facts which mystify him now. 

Mendel's discovery became possible because he worked 
with regular cases of the first category, in which he was able 
to recognize that one of each of the pairs of characters 
he studied did thus prevail and was "dominant" in the 
cross-bred to the exclusion of the other character. This 
fact, which is still an accident of particular cases, Professor 
Weldon, following some of Mendel's interpreters, dignifies 
by the name of the "Law of Dominance," though he 
omits to warn his reader that Mendel states no "Law of 
Dominance " whatever. The whole question whether one or 
other character of the antagonistic pair is dominant though 
of great importance is logically a subordinate one. It 
depends on the specific nature of the varieties and in- 
dividuals used, sometimes probably on the influence of 

118 A Defence of Mendel's 

external conditions and on other factors we cannot now 
discuss. There is as yet no universal law here perceived 
or declared. 

Professor Weldon passes over the proof of the purity 
of the germ-cells lightly enough, but this proposition of 
dominance, suspecting its weakness, he puts prominently 
forward. Briefest equipment will suffice. Facing, as he 
supposes, some new pretender — some local Theudas — 
offering the last crazy prophecy, — any argument will do 
for such an one. An eager gathering in an unfamiliar 
literature, a scrutiny of samples, and he will prove to 
us with small difficulty that dominance of yellow over 
green, and round over wrinkled, is irregular even in peas 
after all ; that in the sharpness of the discontinuity ex- 
hibited by the variations of peas there are many grades ; 
that many of these grades co-exist in the same variety; 
that some varieties may perhaps be normally intermediate. 
All these propositions are supported by the production 
of a collection of evidence, the quality of which we 
shall hereafter consider. "Enough has been said," he 
writes (p. 240), " to show the grave discrepancy between the 
evidence afforded by Mendel's own experiments and that 
obtained by other observers, equally competent and trust- 

We are asked to believe that Professor Weldon has 
thus discovered "a fundamental mistake" vitiating all that 
work, the importance of which, he elsewhere tells us, he 
has "no wish to belittle." 

Principles of Heredity 119 

III. The Facts in regard to Dominance of 
Characters in Peas. 

Professor Weldoii refers to no experiments of his own 
and presumably has made none. Had he done so he would 
have learnt many things about dominance in peas, whether 
of the yellow cotyledon-colour or of the round form, that 
might have pointed him to caution. 

In the year 1900 Messrs Vilmorin-Andrieux & Co. were 
kind enough to send to the Cambridge Botanic Garden on 
my behalf a set of samples of the varieties of Fisum and 
Phaseolus, an exhibit of which had greatly interested me 
at the Paris Exhibition of that year. In the past summer 
I grew a number of these and made some preliminary 
cross-fertilizations among them (about 80 being available 
for these deductions) with a view to a future study of 
certain problems, Mendelian and others. In this work 
I had the benefit of the assistance of Miss Killby of 
Newnham College. Her cultivations and crosses were 
made independently of my own, but our results are almost 
identical. The experience showed me, what a naturalist 
would expect and practical men know already, that a great 
deal turiis on the variety used ; that some varieties are 
very sensitive to conditions while others maintain their 
type sturdily ; that in using certain varieties Mendel's 
experience as to dominance is regularly fulfilled, while in 
the case of other varieties irregularities and even some 
contradictions occur. That the dominance of yellow 
cotyledon-colour over green, and the dominance of the 
smooth form over the wrinkled, is a general truth for 
Pisum sativum appears at once ; that it is a universal 
truth I cannot believe any competent naturalist would 
imagine, still less assert. Mendel certainly never did. 

120 A Defence of Menders 

When he speaks of the "law" or "laws" that he has 
established for Pisuni he is referring to his own discovery 
of the purity of the germ-cells, that of the statistical 
distribution of characters among them, and the statistical 
grouping of the different germ -cells in fertilization, and 
not to the "Law of Dominance" which he never drafted 
and does not propound. 

The issue will be clearer if I here state briefly what, as 
far as my experience goes, are the facts in regard to the 
characters cotyledon-colour and seed-shapes in peas. I have 
not opportunity for more than a passing consideration of 
the seed-coats of pure forms*; that is a maternal character, 
a fact I am not sure Professor Weldon fully appreciates. 
Though that may be incredible, it is evident from many 
passages that he has not, in quoting authorities, considered 
the consequences of this circumstance. 

The normal characters: colour of cotyledons 
and seed-coats. 

Culinary peas (P. sativum, omitting purple sorts) can 
primarily be classified on colour into two groups, yellow^ 
and green. In the green certain pigmentary matters 
persist in the ripe seed which disappear or are decomposed 
in the yellow as the seed ripens. But it may be observed 

* The whole question as to seed-coat colour is most complex. 
Conditions of growth and ripening have a great effect on it. Mr 
Arthur Sutton has shown me samples of Ne Plus Ultra grown in 
England and abroad. This pea has yellow cotyledons with seed-coats 
either yellow or " blue." The foreign sample contained a much 
greater proportion of the former. He told me that generally speaking 
this is the case with samples ripened in a hot, dry climate. 

Unquestionable Xenia appears occasionally, and will be spoken of 
later. Moreover to experiment with such a plant-charsicter an extra 
generation has to be sown and cultivated. Consequently the evidence 
is meagre. 

Principles of Heredity 121 

that the "green" class itself is treated as of two 
divisions, green and blue. In the seedsmen's lists the 
classification is made on the external appearance of the 
seed, without regard to whether the colour is due to the 
seed-coat, the cotyledons, or both. As a rule perhaps 
yellow coats contain yellow cotyledons, and green coats 
green cotyledons, though yellow cotyledons in green coats 
are common, e.g. Gradus, of which the cotyledons are yellow 
while the seed-coats are about as often green as yellow (or 
" white," as it is called technically). Those called "blue" 
consist mostly of seeds which have green cotyledons seen 
through transparent skins, or yellow cotyledons combined 
with green skins. The skins may be roughly classified into 
thin and transparent, or thick and generally at some stage 
pigmented. In numerous varieties the colour of the coty- 
ledon is wholly yellow, or wholly green. Next there are 
many varieties which are constant in habit and other 
properties but have seeds belonging to these two colour 
categories in various proportions. How far these pro- 
portions are known to be constant I cannot ascertain. 

Of such varieties showing mixture of cotyledon-coXoViYii 
nearly all can be described as dimorphic in colour. For 
example in Sutton's Nonpareil Marrowfat the cotyledons 
are almost always either yellow or green, with some piebalds, 
and the colours of the seed-coats are scarcely less distinctly 
dimorphic. In some varieties which exist in both colours 
intermediates are so common that one cannot assert any 
regular dimorphism*. 

* Knowing my interest in this subject Professor Weldon was 
so good as to forward to me a series of bis peas arranged to 
form a scale of colours and shapes, as represented in his Plate I. 
I have no doubt that the use of such colour-scales will much facilitate 
future study of these problems. 

122 A Defence of MendeVs 

There are some varieties which have cotyledons green 
and intermediate shading to greenish yellow, like Stratagem 
quoted by Professor Weldon. Others have yellow and 
intermediate shading to yellowish green, such as McLean's 
Best of all^. I am quite disposed to think there may be 
truly monomorphic varieties with cotyledons permanently 
of intermediate colour only, but so far I have not seen 
onef. The variety with greatest irregularity (apart from 
regular dimorphism) in cotyledon-colour I have seen is a 
sample of ''''mange-tout a rames, a grain vert,^^ but it was a 
good deal injured by weevils {Bruchus), which always cause 
irregularity or change of colour. 

Lastly in some varieties there are many piebalds or 

From what has been said it will be evident that the 
description of a pea in an old book as having been green, 
blue, white, and so forth, unless the cotyledon-colour is 
distinguished from seed-coat colour, needs careful con- 
sideration before inferences are drawn from it. 


In regard to shape, if we keep to ordinary shelling peas, 
the facts are somewhat similar, but as shape is probably 
more sensitive to conditions than cotyledon-colour (not 
than seed-coat colour) there are irregularities to be perhaps 
ascribed to this cause. Broadly, however, there are two 
main divisions, round and wrinkled. It is unquestioned 
that between these two types every intermediate occurs. 

* I notice that Vilmorin in the well-known Plantes Potageres, 
1883, classifies the intermediate- coloured peas with the green. 

t Similarly though tall and dioarf are Mendelian characters, peas 
occur of all heights and are usually classified as tall, half -dwarfs, and 

Princijyles of Heredity 123 

Here again a vast number of varieties can be at once 
classified into round and wrinkled (the classification 
commonly used), others are intermediate normally. Here 
also I suspect some fairly clear sub-divisions might be 
made in the wrinkled group and in the round group too, 
but I would not assert this as a fact. 

I cannot ascertain from botanists what is the nature of 
the difference between round and wrinkled peas, though no 
doubt it will be easily discovered. In maize the round 
seeds contain much unconverted starch, while in the 
wrinkled or sugar-maize this seems to be converted in 
great measure as the seed ripens ; with the result that, 
on drying, the walls collapse. In such seeds we may 
perhaps suppose that the process of conversion, which in 
round seeds takes place on germination, is begun earlier, 
and perhaps the variation essentially consists in the pre- 
mature appearance of the converting ferment. It would be 
most rash to suggest that such a process may be operating 
in the pea, for the phenomenon may have many causes ; 
but however that may be, there is evidently a difference of 
such a nature that when the water dries out of the seed on 
ripening, its walls collapse* ; and this collapse may occur 
in varying degrees. 

* Wrinkling must of course be distinguished further from the 
squaring due to the peas pressing against each other in the pod. 

In connexion with these considerations I may mention that 
Vilmorin makes the interesting statement that most peas retain their 
vitality three years, dying as a rule rapidly after that time is passed, 
though occasionally seeds seven or eight years old are alive; but 
that lorinkled peas germinate as a rule less well than round, and 
do not retain their vitality so long as the round. Vilmoi'in-Andrieux, 
Plantes Potageres, 1883, p. 423. Similar statements regarding the 
behaviour of wrinkled peas in India are made by Firminger, Gardening 
for India, 3rd ed. 1874, p. 146. 

124 A Defence of Mendel's 

In respect of shape the seeds of a variety otherwise 
stable are as a rule fairly uniform, the co-existence of 
both shapes and of intermediates between them in the 
same variety is not infrequent. As Professor Weldon has 
said, Telephone is a good example of an extreme case of 
mixture of both colours and shapes. William /, is another. 
It may be mentioned that regular dimorphism in respect 
of shape is not so common as dimorphism in respect 
of colour. Of great numbers of varieties seen at Messrs 
Suttons' I saw none so distinctly dimorphic in shape as 
William I. which nevertheless contains all grades commonly. 

So far I have spoken of the shapes of ordinary English 
culinary peas. But if we extend our observations to the 
shapes of large-seeded peas, which occur for the most part 
among the sugar-peas (mange-touts), of the "grey" peas 
with coloured flowers, etc., there are fresh complications 
to be considered. 

Professor Weldon does not wholly avoid these (as 
Mendel did in regard to shape) and we will follow him 
through his difliculties hereafter. For the present let me 
say that the classes round and wrinhled are not readily 
applicable to those other varieties and are not so applied 
either by Mendel or other practical writers on these 
subjects. To use the terms indicated in the Introduction, 
seed-shape depends on more than one pair of allelomorphs — 
possibly on several. 

Stability and Variability. 

Generally speaking peas which when seen in bulk are 
monomorphic in colour and shape, will give fairly true and 
uniform offspring (but such strict monomorphism is rather 
exceptional). Instances to the contrary occur, and in my 
own brief experience I have seen some. In a row of Fill- 

Principles of Heredity 125 

basket grown from selected seed there were two plants of 
different habit, seed-shape, etc. Each bore pods with seeds 
few though large and round. Again Blue Peter (blue and 
round) and Laxtoii s Alpha (blue and wrinkled), grown in 
my garden and left to nature uncovered, have each given 
a considerable proportion of seeds with yellow cotyledons, 
about 20 7o ill the case of Laxtoii s Alpha. The distribution 
of these on the plants I cannot state. The plants bearing 
them in each case sprang from green-cotyledoned seeds 
taken from samples containing presumably unselected green 
seeds only. A part of this exceptional result may be due 
to crossing, but heterogeneity of conditions* especially in 
or after ripening is a more likely cause, hypotheses I hope 
to investigate next season. Hitherto I had supposed the 
crossing, if any, to be done by Britchus or Thrips, but 
Tschermak also suspects Megachile, the leaf-cutter bee, 
which abounds in my garden. 

Whatever the cause, these irregularities may undoubtedly 
occur ; and if they be proved to be largely independent of 
crossing and conditions, this will in nowise vitiate the truth 
of the Mendelian principle. For in that case it may simply 
be variability. Such true variation, or sporting, in the 
pea is referred to by many observers. Upon this subject I 
have received most valuable facts from Mr Arthur Sutton, 
who has very kindly interested himself in these inquiries. 

* Cotyledon-colour is not nearly so sensitive to ordinary changes 
in conditions as coat-colour, provided the coat be uninjured. But 
even in monomorphic green varieties, a seed which for any cause has 
burst on ripening, has the exposed parts of its cotyledons ijelloio. 
The same may be the case in seeds of green varieties injured by 
Bruchus or birds. These facts make one hesitate before denying the 
effects of conditions on the cotyledon-colour even of uninjured 
seeds, and the variation described above may have been simply 
weathering. The seeds were gathered very late and man}' were 
burst in Laxton's Alpha. I do not yet know they are alive. 

126 A Defence of MendeVs 

He tells me that several highly bred varieties, selected with 
every possible care, commonly throw a small but constant 
proportion of poor and almost vetch-like plants, with short 
pods and small round seeds, which are hoed out by experi- 
enced men each year before ripening. Other high-class 
varieties always, wherever grown, and when far from other 
sorts, produce a small percentage of some one or more 
definite " sports." Of these peculiar sports he has sent me 
a collection of twelve, taken from as many standard varieties, 
each "sport" being represented by eight seeds, which though 
quite distinct from the type agree with each other in almost 
all cases. 

In two cases, he tells me, these seed-sports sown 
separately have been found to give plants identical with 
the standard type and must therefore be regarded as sports 
in seed characters only ; in other cases change of plant-type 
is associated with the change of seed-tjrpe. 

In most standard varieties these definite sports are not 
ver)^ common, but in a few they are common enough to 
require continual removal by selection"^. 

I hope before long to be able to give statistical details 

* It is interesting to see that in at least one case the same — or 
practically the same — variety has been independently produced by 
different raisers, as we now perceive, by the fortuitous combination 
of similar allelomorphs. Sutton's Ringleader and Carter's First Crop 
(and two others) are cases in point, and it is peculiarly instructive to 
see that in the discussion of these varieties when they were new, one 
of the points indicating their identity was taken to be the fact that 
they produced the same ^^ rogues." See Gard. Chron. 1865, pp. 482 and 
603; 1866, p. 221; 1867, pp. 546 and 712. 

Eimpau quotes Blomeyer [Kultur der Landw. Nutzpfianzen, Leipzig, 
1889, pp. 357 and 380) to the effect that ^wrpZe-flowered plants with 
wrinkled seeds may spring as direct sports from peas with lohite 
flowers and round seeds. I have not seen a copy of Blomeyer's 
work. Probably this "wrinkling" was "indentation." 

Princi2jles of Heredity 127 

and experiments relating to this extraordinarily interesting- 
subject. As de Vries writes in his fine work Die Muta- 
tionstheorie (i. p. 580), " a study of the seed-differences of 
inconstant, or as they are called, ' still ' unfixed varieties, is 
a perfect treasure-house of new discoveries." 

Let us consider briefly the possible significance of these 
facts in the light of Mendelian teaching. First, then, it is 
clear that as regards most of such cases the hypothesis is 
not excluded that these recurring sports may be due to the 
fortuitous concurrence of certain scarcer hypallelomorphs, 
which may either have been free in the original parent 
varieties from which the modern standard forms were 
raised, or may have been freed in the crossing to which the 
latter owe their origin (see p. 28). This possibility raises 
the question whether, if we could make "pure cultures " of 
the gametes, any variations of this nature would ever occur. 
This may be regarded as an unwarrantable speculation, but 
it is not wholly unamenable to the test of experiments. 

But variability, in the sense of division of gonads into 
heterogeneous gametes, may surely be due to causes other 
than crossing. This we cannot doubt. Cross-fertilization 
of the zygote producing those gametes is one of the causes 
of such heterogeneity among them. We cannot suppose it 
to be the sole cause of this phenomenon. 

"When Mendel asserts the purity of the germ-cells of 
cross-breds he cannot be understood to mean that they are 
more pure than those of the original parental races. These 
must have varied in the past. The wrinkled seed arose 
from the round, the green from the yellow (or vice versa, 
if preferred), and probably numerous intermediate forms 
from both. 

The variations, or as I provisionally conceive it, that 
differentiant division among the gametes of which variation 

128 A Defence of MendeVs 

(neglecting environment) is the visible expression, has arisen 
and can arise at one or more points of time, and we have 
no difficulty in believing it to occur now. In many cases 
we have clear evidence that it does. Crossing, — dare we 
call it asymmetrical fertilization ? — is one of the causes of 
the production of heterogeneous gametes — the result of 
divisions qualitatively differentiant and perhaps asjon- 

There are other causes and we have to find them. 
Some years ago I wrote that consideration of the causes 
of variation was in my judgment premature!. Now that 
through Mendel's work we are clearing our minds as to the 
fundamental nature of "gametic" variation, the time is 
approaching when an investigation of such causes maybe 
not unfruitful. 

Of variation as distinct from transmission why does 
Professor Weldon take no heed ? He writes (p. 244) : 

" If Mendel's statements were universally valid, even among 
Peas, the characters of the seeds in the numerous hybrid races 
now existing should fall into one or other of a few definite 
categories, which should not be connected by intermediate 

Now, as I have already pointed out, Mendel made no 
pretence of universal statement : but had he done so, the 
conclusion, which Professor Weldon here suggests should 
follow from such a universal statement, is incorrectly 
drawn. Mendel is concerned with the laws of transmission 

* The asymmetries here conceived may of course be combined in 
an inclusive symmetry. Till the differentiation can be optically 
recognized in the gametes we shall probably get no further with this 
part of the problem. 

t Materials for the Study of Variation, 1894, p. 78. 

Principles of Heredity 129 

of existing characters, not with variation, which he does 
not discuss. 

Nevertheless Professor Weldon has some acquaintance 
with the general fact of variability in certain peas, which 
he mentions (p. 236), but the bearing of this fact on the 
difficulty he enuntiates escapes him. 

Results of crossing in r^egard to seed characters : 
normal and exceptional. 

The conditions being the same, the question of the 
characters of the cross-bred zygotes which we will call 
AB'^ depends primarily on the specific nature of the 
varieties which are crossed to produce them. It is un- 
necessary to point out that if all ^^'s are to look alike, 
both the varieties A and B must be pure — not in the 
common sense of descended, as far as can be traced, 
through individuals identical with themselves, but pure in 
the Mendelian sense, that is to say that each must be at that 
moment producing only homogeneous gametes bearing the 
same characters A and B respectively. Purity of pedigree 
in the breeder's sense is a distinct matter altogether. The 
length of time — or if preferred — the number of generations 
through which a character of a variety has remained pure, 
alters the probability of its dominance, i.e. its appearance 
when a gamete bearing it meets another bearing an antago- 
nistic character, no more, so far as we are yet aware, than 
the length of time a stable element has been isolated alters 
the properties of the chemical compound which may be 
prepared from it. 

Now when individuals (bearing contrary characters), 
pure in the sense indicated, are crossed together, the 
question arises. What will be the appearance of the first 

B. 9 

130 A Defence of MendeVs 

cross individuals ? Here again, generally speaking^ when 
thoroughly green cotyledons are crossed with thoroughly 
yellow cotyledons, the first-cross seeds will have yellow 
cotyledons ; when fully round peas are crossed with fully 
wrinkled the first result will generally speaking be round, 
often with slight pitting as Mendel has stated. This has 
been the usual experience of Correns, Tschermak, Mendel, 
and myself* and, as we shall see, the amount of clear 
and substantial evidence to the contrary is still exceed- 
ingly small. But as any experienced naturalist would 
venture to predict, there is no universal rule in the 
matter. As Professor Weldon himself declares, had there 
been such a universal rule it would surely have been 
notorious. He might further have reflected that in 
Mendel's day, when hybridisation was not the terra 
incognita it has since become, the assertion of such uni- 
versal propositions would have been peculiarly foolish. 
Mendel does not make it ; but Professor Weldon perceiving 
the inherent improbability of the assertion conceives at 
once that Mendel must have made it, and if Mendel 
doesn't say so in words then he must have implied it. 
As a matter of fact Mendel never treats dominance as 
more than an incident in his results, merely using it as 
a means to an end, and I see no reason to suppose he 
troubled to consider to what extent the phenomenon is or 
is not universal — a matter with which he had no concern. 

* The varieties used were Express, Laxton's Alpha, Fillbasket, 
McLean's Blue Peter, Serpette nain blanc, British Queen, tres nain 
de Bretagne, Sabre, mange-tout Debarbieux, and a large "grey" 
sugar-pea, pois sans parchemin geant a tres large cosse. Not counting 
the last two, five are round and three are wrinkled. As to cotyledons, 
six have yellow and four have green. In about 80 crosses I saw no 
exception to dominance of yellow ; but one apparently clear case of 
dominance of wrinkled and some doubtful ones. 

Principles of Heredity 131 

Of course there may be exceptions. As yet we cannot 
detect the causes which control them, though injury, 
impurity, accidental crossing, mistakes of various kinds, 
account for many. Mendel himself says, for instance, that 
unhealthy or badly grown plants give uncertain results. 
Nevertheless there seems to be a true residuum of ex- 
ceptions not to be explained away. I will recite some 
that I have seen. In my own crosses I have seen green x 
gTeen give yellow four times. This I incline to attribute 
to conditions or other disturbance, for the natural pods of 
these plants gave several yellows. At Messrs Suttons' I saw 
second-generation seeds got by allowing a cross of Sutton's 
Centenary (gr. wr.) x Eclipse (gr. rd.) to go to seed ; the 
resulting seeds were both green and yellow, wrinkled and 
round. But in looking at a sample of Eclipse I found 
a few yellow seeds, say two per cent., which may perhaps 
be the explanation. Green wrinkled x green round may 
give all wrinkled, and again \vrinkled x wrinkled may give 
round'^. Of this I saw a clear case — supposing no mistake 
to have occurred — at Messrs Suttons'. Lastly we have 
the fact that in exceptional cases crossing two forms — 
apparently pure in the strict sense — may give a mixture 
in the first generation. There are doubtless examples also 
of unlikeness between reciprocals, and of this too I have 
seen one putative caset. 

Such facts thus set out for the first cross-bred 
generation may without doubt be predicated for subsequent 

What then is the significance of the facts ? 

* Professor Weldon may take this as a famous blow for Mendel, 
till he realizes what is meant by Mendel's '* Hybrid-character." 

t In addition to those spoken of later, where the great difference 
between reciprocals is due to the maternal characters of the seeds. 


132 A Defence of MendeVs 

A nalysis of exceptions. 

Assuming that all these "contradictory" phenomena 
happened truly as alleged, and were not pathological or 
due to error — an explanation which seems quite inadequate 
— there are at least four possible accounts of such diverse 
results — each valid, without any appeal to ancestry. 

1. That dominance may exceptionally fail — or in other 
words he created on the side which is elsewhere recessive. 
For this exceptional failure we have to seek exceptional 
causes. The artificial creation of dominance (in a character 
usually recessive) has not yet to my knowledge been demon- 
strated experimentally, but experiments are begun by which 
such evidence may conceivably be obtained. 

2. There may be what is known to practical students 
of evolution as the false hybridism of Millardet, or in other 
words, fertilisation with — from unknown causes — transmis- 
sion of none or of only some of the characters of one pure 
parent. The applicability of this hypothesis to the colours 
and shapes of peas is perhaps remote, but we may notice that 
it is one possible account of those rare cases where two 
pure forms give a mixed result in the first generation, even 
assuming the gametes of each pure parent to be truly 
monomorphic as regards the character they bear. The 
applicability of this suggestion can of course be tested by 
study of the subsequent generations, self-fertilised or ferti- 
lised by similar forms produced in the same way. In the 
case of a genuine false-hybrid the lost characters will not 
reappear in the posterity. 

3. The result may not be a case of transmission at all 
as it is at present conceived, but of the creation on crossing 

Princijyles of Hei^ecUty 1 33 

of something new. Our ^^'s may have one or more 
characters ijeculiar to themselves. We may in fact have 
made a distinct " mule " or heterozygote form. Where this 
is the case, there are several subordinate possibilities we 
need not at present pursue. 

4. There may be definite variation (distinct from that 
proper to the "mule") consequent on causes we cannot 
yet surmise (see pp. 125 and 128). 

The above possibilities are 1 believe at the present time 
the only ones that need to be considered in connexion with 
these exceptional cases*. They are all of them capable 
of experimental test and in certain instances we are 
beginning to expect the conclusion. 

The " mule " or heterozygote. 

There can be little doubt that in many cases it is to 
the third category that the phenomena belong. An indication 
of the applicability of this reasoning will generally be found 
in the fact that in such "mule" forms the colour or the 
shape of the seeds will be recognizably peculiar and proper 
to the specimens themselves, as distinct from their parents, 
and we may safely anticipate that when those seeds are 
grown the plants will show some character which is 
recognizable as novel. The proof that the reasoning may 
apply can as yet only be got by finding that the forms in 

* I have not here considered the case in which male and female 
elements of a pure variety are not homolofjous and the variety is a 
permanent monomorphic " mule." Such a phenomenon, when present, 
will prove itself in reciprocal crossing. I know no such case in 
peas for certain. 

134 A Defence of MendeVs 

question cannot breed true even after successive selections, 
but constantly break up into the same series of forms*. 

This conception of the "mule" form, or "hybrid- 
character" as Mendel called it, though undeveloped, is 
perfectly clear in his work. He says that the dominant 
character may have two significations, it may be either a 
parental character or a hybrid-character, and it must be 
differentiated according as it appears in the one capacity 
or the other. He does not regard the character displayed 
by the hybrid, whether dominant or other, as a thing 
inherited from or transmitted by the pure parent at all, but 
as the peculiar function or property of the hybrid. When 
this conception has been fully understood and appreciated 
in all its bearings it will be found to be hardly less fruitful 
than that of the purity of the germ-cells. 

The two parents are two — let us say — substances f 
represented by corresponding gametes. These gametes 
unite to form a new "substance" — the cross-bred zj-gote. 
This has its own j)roperties and structure, just as a chemical 
compound has, and the properties of this new " substance " 
are not more strictly traceable to, or "inherited" from, 
those of the two parents than are those of a new chemical 
compound "inherited" from those of the component 
elements. If the case be one in which the gametes are 
pure, the new " substance " is not represented by them, 
but the compound is again dissociated into its components, 
each of which is separately represented by gametes. 

* It will be understood that a " mule " form is quite distinct from 
what is generally described as a "blend." One certain criterion of 
the " mule " form is the fact that it cannot be fixed, see p. 25. 
There is little doubt that Laxton had such a " mule " form when he 
speaks of "the remarkably fine but unfixable pea, Evolution." J. R. 
Hort. Soc. XII. 1890, p. 37 {v. infra). 

t Using the word metaphorically. 

Principles of Heredity 135 

The character of the cross-bred zygote may be anything. 
It may be something we have seen before in one or other of 
the parents, it may be intermediate between the two, or it 
may be something new. All these possibilities were known 
to Mendel and he is perfectly aware that his principle is 
equally applicable to all. The first case is his " dominance." 
That he is ready for the second is sufficiently shown by his 
brief reference to time of flowering considered as a character 
(p. &o). The hybrids, he says, flower at a time almost 
exactly intermediate between the flowering times of the 
parents, and he remarks that the development of the 
hybrids in this case probably happens in the same way as 
it does in the case of the other characters*. 

That he was thoroughly prepared for the third possibility 
appears constantly through the paper, notably in the 
argument based on the Phaseolus hybrids, and in the 
statement that the hybrid between tails and dwarfs is 
generally taller than the tall parent, having increased 
height as its " hybrid-character." 

All this Professor Weldon lias missed. In place of it 
he off'ers us the sententia that no one can expect to 
understand these phenomena if he neglect ancestry. This 
is the idle gloss of the scribe, which, if we erase it not 
thoroughly, may pass into the text. 

Enough has been said to show how greatly Mendel's 

■ conception of heredity was in advance of those which 

pass current at the present day ; I have here attempted 

* " Ueber die BlUthezeit der Hybriden sind die Versuche noch jiicht 
abfjeschlossen. So viel kann indessen schon anrjecjeben werden, dass 
diescdbe fast genau in der Mitte zwischen jener der Samen- nnd 
Pollenpjianze steht, und die Enhvicklmig der Hybriden bezuglich 
dieses Merkviales xcaltrscheinlich in der namlichen Weise erfolgt, loie es 
fiir die iibriyen Merkmale der Fall ist." Mendel, p. 23. 

136 A Defence of MendeVs 

the barest outline of the nature of the "hybrid-character," 
and I have not sought to indicate the conclusions that we 
reach when the reasoning so clear in the case of the hybrid 
is applied to the pure forms and their own characters. 

In these considerations we reach the very base on which 
all conceptions of heredity and variation must henceforth 
rest, and that it is now possible for us to attempt any such 
analysis is one of the most far-reaching consequences of 
Mendel's principle. Till two years ago no one had made 
more than random soundings of this abyss. 

I have briefly discussed these possibilities to assist the 
reader in getting an insight into Mendel's conceptions. 
But in dealing with Professor Weldon we need not make 
this excursion ; for his objection arising from the absence of 
uniform regularity in dominance is not in point. 

The soundness of Mendel's work and conclusions would 
be just as complete if dominance be found to fail often 
instead of rarely. For it is perfectly certain that varieties 
can be chosen in such a way that the dominance of one 
character over its antagonist is so regular a phenomenon 
that it can be used in the way Mendel indicates. He chose 
varieties, in fact, in which a known character was regularly 
dominant and it is because he did so that he made his 
discovery^. When Professor Weldon speaks of the exist- 
ence of fluctuation and diversity in regard to dominance as 
proof of a " grave discrepancy " between Mendel's facts and 
those of other observers!, he merely indicates the point at 
which his own misconceptions began. 

* As has been already shown the discovery could have been 
made equally well and possibly with greater rapidity in a case in 
which the hybrid had a character distinct from either parent. The 
cases that would not have given a clear result are those where there 
is irregular dominance of one or other parent. 

t Weldon, p. 240. 

Principles of Heredity 137 

From Mendel's style it may be inferred that if he had 
meant to state universal dominance in peas he would 
have done so in unequivocal language. Let me point out 
further that of the 34 varieties he collected for study, he 
discarded 12 as not amenable to his purposes*. He tells 
us he would have nothing to do with characters which 
were not sharp, but of a " more or less " description. As 
the 34 varieties are said to have all come true from seed, 
we may fairly suppose that the reason he discarded twelve 
was that they were unsuitable for his calculations, having 
either ill-defined and intermediate characters, or possibly 
defective and irregular dominance. 

IV. Professor Weldon's collection of " Other 


A. In regard to cotyledon colour: Preliminary. 

I have been at some pains to show how the contradictory 
results, no doubt sometimes occurring, on which Professor 
Weldon lays such stress, may be comprehended without 
any injury to Mendel's main conclusions. This excursion 
was made to save trouble with future discoverers of 
exceptions, though the existence of such facts need 
scarcely disturb many minds. As regards the dominance 
of yellow cotyledon-colour over green the whole number of 
genuine unconformable cases is likely to prove very small 
indeed, though in regard to the dominance of round shape 
over wrinkled we may be prepared for more discrepancies. 
Indeed my own crosses alone are sufficient to show that 
in using some varieties irregularities are to be expected. 

* See p. 43. 

138 A Defence of MendeVs 

Considering also that the . shapes of peas depend un- 
questionably on more than one pair of allelomorphs I 
fully expect regular blending in some cases. 

As however it may be more satisfactory to the reader 
and to Professor Weldon if I follow him through his 
"contradictory" evidence I Avill endeavour to do so. Those 
who have even a slight practical acquaintance with the 
phenomena of heredity will sympathize with me in the 
difficulty I feel in treating this section of his arguments 
with that gravity he conceives the occasion to demand. 

In following the path of the critic it will be necessary 
for me to trouble the reader with a number of details of a 
humble order, but the journey will not prove devoid of 

Now exceptions are always interesting and suggestive 
things, and sometimes hold a key to great mysteries. Still 
when a few exceptions are found disobeying rules elsewhere 
conformed to by large classes of phenomena it is not an 
unsafe course to consider, with such care as the case permits, 
whether the exceptions may not be due to exceptional 
causes, or failing such causes whether there may be any 
possibility of error. But to Professor Weldon, an exception 
is an exception — and as such may prove a very serviceable 
missile ; so he gathers them as they were "smooth stones 
from the brook." 

Before examining the quality of this rather miscellaneous 
ammunition I would wish to draw the non-botanical reader's 
attention to one or two facts of a general nature. 

For our present purpose the seed of a pea may be 
considered as consisting of two parts, the embryo with its 
cotyledons, enclosed in a seed-coat. It has been known for 
about a century that this coat or skin is a maternal structure, 
being part of the mother plant just as much as the pods 

Principles of Heredity 139 

are, and consequently not belonging to the next generation 
at all. If then any changes take place in it consequent on 
fertilisation, they are to be regarded not as in any sense a 
transmission of character by heredity, but rather as of the 
nature of an "infection." If on the other hand it is desired 
to study the influence of hereditary transmission on seed- 
coat characters, then the crossed seeds must be sown and 
the seed-coats of their seeds studied. Such infective changes 
in maternal tissues have been known from early times, a 
notable collection of them having been made especially by 
Darwin ; and for these cases Focke suggested the convenient 
word Xenia. With this familiar fact I would not for a 
moment suppose Professor Weldon unacquainted, though it 
was with some surprise that I found in his paper no reference 
to the phenomenon. 

For as it happens, xenia is not at all a rare occurrence 
with certain 'varieties of peas ; though in them, as I believe 
is generally the case with this phenomenon, it is highly 
irregular in its manifestations, being doubtless dependent 
on slight differences of conditions during ripening. 

The coats of peas differ greatly in different varieties, 
being sometimes thick and white or yellow, sometimes 
thick and highly pigmented with green or other colours, 
in both of which cases it may be impossible to judge the 
cotyledon-colour without peeling off the opaque coat ; or 
the coats may be very thin, colourless and transparent, so 
that the cotyledon-colour is seen at once. It was such a 
transparent form that Mendel says he used for his experi- 
ments with cotyledon-colour. In order to see xenia a pea 
with a pigmented seed-coat should be taken as seed-parent, 
and crossed with a variety having a different cotyledon- 
colour. There is then a fair chance of seeing this 
phenomenon, but much still depends on the variety. For 

140 A Defence of MendeVs 

example, Fillbasket has green cotyledons and seed- coat 
green except near the hilar surface. Crossed with BerpetU 
nain blanc (yellow cotyledons and yellow coat) this variety 
gave three pods with 17 seeds in which the seed-coats were 
almost full yellow (xenia). Three other pods (25 seeds), 
similarly produced, showed slight xenia, and one pod with 
eight seeds showed little or none. 

On the other hand Fillbasket fertilised with nain de 
Bretagne (yellow cotyledons, seed-coats yellow to yellowish 
green) gave six pods wdth 39 seeds showing slight xenia, 
distinct in a few seeds but absent in most. 

Examples of xenia produced by the contrary proceeding, 
namely fertilising a yellow pea with a green, may indubitably 
occur and I have seen doubtful cases ; but as by the nature 
of the case these are negative phenomena, i.e. the seed-coat 
remaining greenish and not going through its normal 
maturation changes, they must always be equivocal, and 
would require special confirmation before other causes were 

Lastly, the special change (xenia) Mendel saw in "grey" 
peas, appearance or increase of purple pigment in the thick 
coats, following crossing, is common but also irregular. 

If a transparent coated form be taken as seed-parent 
there is no appreciable xenia, so far as I know, and such a 
phenomenon would certainly be paradoxical*. 

In this connection it is interesting to observe that 
Giltay, whom Professor Weldon quotes as having obtained 
purely Mendelian results, got no xenia though searching 
for it. If the reader goes carefully through Giltay's 
numerous cases, he will find, almost without doubt, that 
none of them were such as produce it. Reading Giant, as 

* In some transparent coats there is pigment, but so little as a 
rule that xenia would be scarcely noticeable. 

Pmnciples of Heredity 141 

Giltay states, has a transjparent skin, and the only xenia 
likely to occur in the other cases would be of the peculiar 
and uncertain kind seen in using "grey" peas. Professor 
AYeldon notes that Giltay, who evidently worked with ex- 
treme care, peeled his seeds before describing them, a course 
which Professor Weldon, not recognizing the distinction 
between the varieties with opaque and transparent coats, 
himself wisely recommends. The coincidence of the peeled 
seeds giving simple Mendelian results is one which might 
have alarmed a critic less intrepid than Professor Weldon. 

Bearing in mind, then, that the coats of peas may be 
transparent or opaque ; and in the latter case may be 
variously pigmented, green, grey, reddish, purplish, etc. ; 
that in any of the latter cases there may or may not be 
xenia ; the reader will perceive that to use the statements 
of an author, whether scientific or lay, to the effect that on 
crossing varieties he obtained peas of such and such colours 
without specifying at all whether the coats were transparent 
or whether the colours he saw were coat- or cotyledon-colours 
is a proceeding fraught with peculiar and special risks. 

(1) Gartner s cases. Professor Weldon gives, as ex- 
ceptions, a series of Gartner's observations. Using several 
varieties, amongst them Pisum sativum macrospermum, 
a "grey" pea, mth coloured flowers and seed-coats*, 
he obtained results partly Mendelian and partly, as 
now alleged, contradictory. The latter consist of seeds 
" dirty yellow " and " yellowish green," whereas it is 
suggested they should have been simply yellow. 

Now students of this department of natural history will 

know that these same observations of Gartner's, whether 

rightly or wrongly, have been doing duty for more than 

half a century as stock illustrations of xenia. In this 

* Usually correlated characters, as Mendel knew. 

142 A Defence of MendeVs 

capacity they have served two generations of naturalists. 
The ground nowadays may be unfamiliar, but others have 
travelled it before and recorded their impressions. Darwin, 
for example, has the following passage"^ : 

"These statements led Gartner, who was highly sceptical on 
the subject, carefully to try a long series of experiments ; he 
selected the most constant varieties, and the results conclusively 
showed that the colour of the shin of the pea is modified when 
pollen of a differently coloured variety is used." (The italics are 

In the true spirit of inquiry Professor Weldon doubtless 

'"Tis not Antiquity nor Author^ 
That makes Truth Truth, altho' Time's Daughter ^^ ; 

but perhaps a word of caution to the reader that another 
interpretation exists would have been in place. It cannot 
be without amazement therefore that we find him appro- 
priating these examples as referring to cotyledon-colour, 
with never a hint that the point is doubtful. 

Giltay, without going into details, points out the 
ambiguity!. As Professor Weldon refers to the writings 
both of Darwin and Giltay, it is still more remarkable 
that he should regard the phenomenon as clearly one of 
cotyledon-colour and not coat-colour as Darwin and many 
other writers have supposed. 

* Animals and Plants, 2nd ed. 1885, p. 428. 

+ " Eine andere Frage ist jedoch, oh der Einfluss des Pollens auf 
den Keini schon dusserlich an diesen letzteren sichtbar sein kann. 
Darwin fuhrt mehrere hierher gehorige Fdlle an, und ivahrscheinlich 
sind auch die Resultate der von Gartner ilber diesen Gegenstand aus- 
gefiihrten Experimente hier zu erwdhnen, ivenn es auch nicht ganz 
deutlich ist, oh der von Gartner erwdhnte directe Einfluss des Pollens 
sich nur innerhalh der Grenzen des Keimes merklich macht oder nicht.^^ 
p. 490. 

Principles of Heredity 143 

Without going further it would be highly improbable 
that Gartner is speaking solely or even chiefly of the 
cotyledons, from the circumstance that these observations 
are given as evidence of " the influence of foreign pollen on 
the female organs^^ ; and that Gartner was perfectly aware of 
the fact that the coat of the seed was a maternal structure 
is evident from his statement to that effect on p. 80. 

To go into the whole question in detail would require 
considerable space ; but indeed it is unnecessary to labour 
the point. The reader who examines Gartner's account 
with care, especially the peculiar phenomena obtained in 
the case of the "gxey" pea {macrospermuni), with specimens 
before him, will have no difficulty in recognizing that 
Gartner is simply describing the seeds as they looked in 
their coats, and is not attempting to distinguish cotyledon- 
characters and coat-characters. If he had peeled them, 
which in the case of "grey" peas would be absolutely 
necessary to see cotyledon-colour, he must surely have 
said so. 

Had he done so, he would have found the cotyledons 
full yellow in every ripe seed ; for I venture to assert that 
anyone who tries, as we have, crosses between a yellow- 
cotyledoned "grey" pea, such as Gartner's was, with any 
pure green variety will see that there is no question 
whatever as to absolute dominance of the yellow cotyledon- 
character here, more striking than in any other case. 
If exceptions are to be looked for, they will not be found 
there ; and, except in so far as they show simple dominance 
of yellow, Gartner's observations cannot be cited in this 
connection at all. 

(2) Seton's case. Another exception given by Pro- 
fessor Weldon is much more interesting and instructive. 

144 A Defence of MendeFs 

It is the curious case of Seton"^. Told in the words of 
the critic it is as follows : — 

" Mr Alexander Seton crossed the flowers of Dwarf Imperial^ 
'a well-known green variety of the Pea,' with the pollen of 
'a white free-growing variety.' Four hybrid seeds were ob- 
tained, ' which did not differ in appearance from the others 
of the female parent.' These seeds therefore did not obey the 
law of dominance, or if the statement be preferred, greenness 
became dominant in this case. The seeds were sown, and 
produced plants bearing ' green ' and ' white ' seeds side by 
side in the same pod. An excellent coloured figure of one of 
these pods is given {loc. cit. Plate 9, Fig. 1), and is the only 
figure I have found which illustrates segregation of colours in 
hybrid Peas of the second generation." 

Now if Professor Weldon had applied to this case the 
same independence of judgment he evinced in dismissing 
Darwin's interpretation of Gartner's observations, he might 
have reached a valuable result. Knowing how difficult it 
is to give all the points in a brief citation, I turned up the 
original passage, where I find it stated that the mixed 
seeds of the second generation " were all completely either 
of one colour or the other, none of them having an inter- 
mediate tint, as Mr Seton had expected." The utility of 
this observation of the absence of intermediates, is that it 
goes some way to dispose of the suggestion of xenia as a 
cause contributing to the result. 

Moreover, feeling perfectly clear, from the fact of the 
absence of intermediates, that the case must be one of 
simple dominance in spite of first appearances, I suggest 
the following account with every confidence that it is 
the true one. There have been several ^'' Imperials ^^ 

* Appendix to paper of Goss, Trans. Hort. Soc. v. 1822, pub. 
1824 {not 1848, as given by Professor "Weldon), p. 236. 

Principles of Heredity 145 

though Dwarf Imperial, in a form which I can feel sure 
is Seton's form, I have not succeeded in seeing ; but 
from Vilmorin's description that the peas when ripe are 
^franchement verts^^ I feel no doubt it was a green pea 
with a green skin. If it had had a transparent skin this 
description would be inapplicable. Having then a green 
skin, which may be assumed with every probability of truth, 
the seeds, even though the cotyledons were yellow, might, 
especially if examined fresh, be indistinguishable from those 
of the maternal type. Next from the fact of the mixture 
in the second generation we learn that the semi-transparent 
seed-coat of the paternal form was dominant as a plant- 
character, and indeed the coloured plate makes this fairly 
evident. It will be understood that this explanation is 
as yet suggestive, but from the facts of the second genera- 
tion, any supposition that there was real irregularity in 
dominance in this case is out of the question*. 

(3) Tschermak's exceptions. These are a much more 
acceptable lot than those we have been considering. 
Tschermak was thoroughly alive to the seed-coat question 
and consequently any exception stated as an unqualified 
fact on his authority must be accepted. The nature of these 
cases we shall see. Among the many varieties he used, 
some being not monomorphic, it would have been sur- 
prising if he had not found true irregularities in dominance. 

(3 a) Buchsbaum case. This variety, growing in the 
open, gave once a pod in which every seed hut one was green. 
In stating this case Professor Weldon refers to Buchsbaum 

* Since the above passage was written I find the " Imperials ^^ 
described in "Report of Chiswick Trials," Proc. R. Hort. Soc. 1860, 
I. p. 340, as "skin thick " ; and on p. 360 " skin thick, blue " ; which 
finally disposes of this " exception." 

B. 10 

146 A Defence of MendeVs 

as "a yellow-seeded variety." Tschermak"^, however, de- 
scribes it as having '^ gelbes, qfters gelblich-grunes Speicker- 
gewehe " (cotyledons) ; and again says the cotyledon-colour 
is ^'' aller dings gerade hei Buchsbaum zur Spontanvariation 
nach gelb-grun neigend!" The (!) is Tschermak's. There- 
fore Professor Weldon can hardly claim Buchsbaum as 
"yellow-seeded" without qualification. 

Buchsbaum in fact is in all probability a blend-form 
and certainly not a true, stable yellow. One of the green 
seeds mentioned above grew and gave 15 yellows and three 
greens, and the result showed pretty clearly, as Tschermak 
says, that there had been an accidental cross with a tall 

On another occasion Telephone ? (another impure 
green) x Buchsbaum gave four yellow smooth and two green 
wrinkled, but one [?both: the grammar is obscure] of the 
greens did not germinate!. 

(3 b) Telephone cases. Telephone, crossed with at least 
one yellow variety (Auvergne) gave all or some green or 
greenish. These 1 have no doubt are good cases of 
" defective dominance " of yellow. But it must be noted 
that Telephone is an impure green. Nominally a green, it 
is as Professor Weldon has satisfied himself, very irregular 
in colour, having many intermediates shading to pure yellow 
and many piebalds. It is the variety from which alone 
Professor Weldon made his colour-scale. / desire therefore 
to call special attention to the fact that Telephone, though 

* (36), p. 502 and (37), p. 663. 

t Professor Weldon should have alluded to this. Dead seeds 
have no bearing on these questions, seeing that their characters may 
be pathological. The same seeds are later described as " wie 
Telephone selbst," so, apart from the possibility of death, they may 
also have been self-fertilised. 

Principles of Heiedity 147 

not a pure green^ Tschermak^s sample being as he says 
"" gelhlichweiss gr'im^' a yellowisJi-icliite-green in cotyledon- 
colour^ is the variety which has so far contributed the 
cleai^est evidence of the green colour dominating in its 
crosses with a yellow \ and that Buchsbaum is probably a 
similar case. To tbis point we sball return. It may not 
be superfluous to mention also that one cross between 
Fillbasket (a thorough green) and Telephone gave three 
yellowish green seeds (Tschermak, (36), p. 501). 

(3 c) Couturier cases. This fully yellow variety in 
crosses with two fully green sorts gave seeds either yellow 
or greenish yellow. In one case Fillbasket ? fertilised by 
Couturier gave mixed seeds, green and yellow. For any 
evidence to the contrary, the green in this case may have 
been self-fertilised. Nevertheless, taking the evidence 
together, I think it is most likely that Couturier is a 
genuine case of imperfect dominance of yellow. If so, it is 
the only true "exception" in crosses between stable forms. 

We have now narrowed down Professor Weldon's 
exceptions to dominance of cotyledon-colour to two varieties, 
one yellow {Couturier), and one yellow "tending to green" 
{Buchsbaum), which show imperfect dominance of yellow ; 
and one variety. Telephone, an impure and irregular green, 
which shows occasional but uncertain dominance of green. 

What may be the meaning of the phenomenon shown 
by the unstable or mosaic varieties we cannot tell ; but I 
venture to suggest that when we more fully appreciate the 
nature and genesis of the gametes, it will be found that 
the peculiarities of heredity seen in these cases have more 
in common with those of ''false hybridism" (see p. 34) 
than with any true failure of dominance. 

Before, however, feeling quite satisfied in regard even 


148 A Defence of MendeVs 

to this residuum of exceptions, one would wish to learn 
the subsequent fate of these aberrant seeds and how their 
offspring differed from that of their sisters. One only of 
them can I yet trace, viz. the green seed from Telephone ? 
X Buchshaum 3 , which proved a veritable "green dominant." 
As for the remainder, Tschermak promises in his first 
paper to watch them. But in his second paper the only 
passage I can find relating to them declares that perhaps 
some of the questionable cases he mentioned in his first 
paper ^^ are attributable to similar isolated anomalies in 
dominance; some proved themselves by subsequent cultivation 
to be cases of accidental self -fertilisation ; others failed to 
germinate^ y I may warn those interested in these ques- 
tions, that in estimating changes due to ripening, dead 
seeds are not available. 

B. Seed-coats and shapes. 

1. Seed-coats. Professor Weldon lays some stress on 
the results obtained by Correnst in crossing a pea having 
green cotyledons and a thin almost colourless coat {grune 
spate Erfurter Folger-erbse) with two purple-flowered 
varieties. The latter are what are known in England 
as "grey" peas, though the term grey is not generally 

In these varieties the cotyledon-colour is yellow and 

* '■^Vielleicht sind einige der I.e. 507 bis 508 erwdhnten fraglichen 
Fdlle auf dhnliche vereinzelte Anomalien der MerJcmalswerthigkeit 
zu beziehen ; einige eriviesen sich allerdings beini Anbau ah Producte 
ungeioollter Selbstbefruchtung, andere keimten nicht." 

t Kegarding this case I have to thank Professor Correns for a 
good deal of information which he kindly sent me in response to my 
inquiry. I am thus able to supplement the published account in 
'■some particulars. 

Princiijles of Heredity 149 

the coats are usually higlily coloured or orange-brown. 
In reciprocal crosses Correns found no change from the 
maternal seed-coat-colour or seed-shape. On sowing these 
peas he obtained plants bearing peas which, using the 
terminology of Mendel and others, he speaks of as the "first 

These peas varied in the colour of their seed-coats 
from an almost colourless form slightly tinged with green 
like the one parent to the orange-brown of the other 
parent. The seeds varied in this respect not only from 
plant to plant, but from pod to pod, and from seed to seed, 
as Professor Correns has informed me. 

The peas with more highly-coloured coats were sown and 
gave rise to plants with seeds showing the whole range of 
seed-coat-colours again. 

Professor Weldon states that in this case neither the 
law of dominance nor the law of segregation was observed ; 
and the same is the opinion of Correns, who, as I under- 
stand, inclines to regard the colour-distribution as indi- 
cating a "mosaic" formation. This is perhaps conceiv- 
able ; and in that case the statement that there was no 
dominance would be true, and it would also be true that 
the unit of segregation, if any, was smaller than the in- 
dividual plant and may in fact be the individual seed. 

A final decision of this question is as yet impossible. 
Nevertheless from Professor Correns I have learnt one 
point of importance, namely, that the coats of all these 
seeds were thick, like that of the coloured and as usual 
dominant form. There is no "mosaic" of coats like one 
parent and coats like the other, though there may be a 
mosaic of colours. In regard to the distribution of colour 
however the possibility does not seem to me excluded that 
we are here dealing with changes influenced by conditions. 

150 A Defence of MendeVs 

I have grown a "grey " pea and noticed that the seed-coats 
ripened in my garden differ considerably and not quite 
uniformly from those received from and probably ripened 
in France, mine being mostly pale and greyish, instead 
of reddish-brown. We have elsewhere seen (p. 120) that 
pigments of the seed-coat-colour may be very sensitive to 
conditions, and slight differences of moisture, for example, 
may in some measure account for the differences in colour. 
Among my crosses I have a pod of such " grey " peas ferti- 
lised hy Laxton s Alpha (green cotyledons, coat transparent). 
It contained five seeds, of which four were red-hrown on 
one side and grey with purple specks on the other. The 
fifth was of the grey colour on both sides. I regard this 
difference not as indicating segregation of character but 
merely as comparable with the difference between the two 
sides of a ripe apple, and I have little doubt that Correns' 
case may be of the same nature"^. Phenomena somewhat 
similar to these will be met with in Laxton's case of the 
*' maple" seeded peas (see p. 161). 

2. Seed-shapes. Here Professor Weldon has three sets 
of alleged exceptions to the rule of dominance of round 
shape over wrinkled. The first are Rimpau's cases, the 
second are Tschermak's cases, the third group are cases of 
*' grey " peas, which we will treat in a separate section (see 
pp. 153 and 158). 

{a) Rimpau's cases. Professor Weldon quotes Rimpau 
as having crossed wrinkled and round peasf and found 

* Mr Hurst, of Burbage, tells me that in varieties having coats 
green or white, e.g. American Wonder, the white coats are mostly 
from early, the green from later pods, the tints depending on 
conditions and exposm'e. 

t In the first case KnighVs MaiToiv with Victoria, both ways ; in 
the second Victoria with Telephone, both ways. 

Principles of Heredity 151 

the second hybrid generation dimorphic as usual. The 
wrinkled peas were selected and sown and gave wrinkled 
peas and round peas, becoming "true" to the wrinkled 
character in one case only in the fifth year, while in the 
second case — that of a Telephone cross — there was a mixture 
of round and wrinkled similarly resulting from wrinkled 
seed for two years, but the experiment was not continued. 

These at first sight look like genuine exceptions. In 
reality, however, they are capable of a simple explanation. It 
must be remembered that Kimpau was working in ignorance 
of Mendel's results, was not testing any rule, and was not 
on the look out for irregularities. Now all who have 
crossed wrinkled and round peas on even a moderate scale 
will have met with the fact that there is frequently some 
wrinkling in the cross-bred seeds. Though round when com- 
pared with the true wrinkled, these are often somewhat more 
wrinkled than the round type, and in irregular degrees. 
For my own part I fully anticipate that we may find rare 
cases of complete blending in this respect though I do not 
as yet know one. 

Rimpau gives a photograph of eight peas (Fig. 146) 
which he says represent the wrinkled form derived from 
this cross. It is evident that these are not from one pod 
but a miscellaneous selection. On close inspection it will 
be seen that while the remainder are shown with their 
coti/ledo7i-sur{&ce^ upwards, the two peas at the lower end 
of the row are represented with their hilar-snYhces 
upwards. Remembering this it will be recognized that 
these two lower peas are in fact not fully wrinkled peas 
but almost certainly round " hybrids," and the depression 
is merely that which is often seen in round peas (such as 
Fillbasket), squared by mutual pressure. Such peas, when 
sown, might of course give some round. 

152 A Defence of MendeVs 

As Tschermak writes ((37), p. 658), experience has 
shown him that cross-bred seeds with character transitional 
between "round" and "wrinkled" behave as hybrids, and 
have both wrinkled and round offspring, and he now reckons 
them accordingly with the round dominants. 

Note further the fact that Rimpau found the wrinkled 
form came true in the fifth year, while the round gave at 
first more, later fewer, wrinkleds, not coming true till the 
ninth year. This makes it quite clear that there was 
dominance of the round form, but that the heterozygotes 
were not so sharply distinguishable from the two pure 
forms as to be separated at once by a person not on the 
look-out for the distinctions. Nevertheless there was 
sufficient difference to lead to a practical distinction of 
the cross-breds both from the pure dominants and from 
the pure recessives. 

The Telephone case may have been of the same nature ; 
though, as we have seen above, this pea is peculiar in its 
colour-heredity and may quite well have followed a different 
rule in shape also. As stated before, the wrinkled off- 
spring were not cultivated after the third 3^ear, but the 
round seeds are said to have still given some wrinkleds in 
the eighth year after the cross, as would be expected in a 
simple Mendelian case. 

(h) TschermaFs cases. The cases Professor Weldon 
quotes from Tschermak all relate to crosses with Telephone 
again, and this fact taken with the certainty that the 
colour-heredity of Telephone is abnormal makes it fairly 
clear that there is here something of a really exceptional 
character. What the real nature of the exception is, and 
how far it is to be taken as contradicting the "law of 
dominance," is quite another matter. 

Principles of Heredity 153 

3. Other phenomena, especially regarding seed-shapes, 
in the case of ^^ grey^^ peas. Modern evidence. Professor 
Welclon quotes from Tschermak the interesting facts about 
the "grey" pea, Graue Rlesen, but does not attempt to 
elucidate them. He is not on very safe ground in adducing 
these phenomena as conflicting with the "law of dominance." 
Let us see whither we are led if we consider these cases. 
On p. 124 I mentioned that the classes round and wrinkled 
do not properly hold if we try to extend them to large- 
seeded sorts, and that these cases require separate con- 
sideration. In many of such peas, which usually belong 
either to the classes of sugar-peas {mange-touts) or " grey " 
peas (with coloured flowers), the seeds would be rather 
described as irregularly indented, lumpy or stony*, than by 
any use of the terms round or wrinkled. One sugar-pea 
{Debarhieux) which I have used has large flattish, smooth, 
yellow seeds with white skins, and this also in its crossings 
follows the rules about to be described for the large-seeded 
"grey "peas. 

In the large "grey" peas the most conspicuous feature 
is the seed-coat, which is grey, brownish, or of a bright 
reddish colour. Such seed-coats are often speckled with 
purple, and on boiling these seed-coats turn dark brown. 
They are in fact the very peas used by Mendel in making 
up his third pair of characters. Regarding them Professor 

* Gartner's macrospermum was evidently one of these, though 
from the further account (p. 498) it was probably more wrinkled. 
There are of course mange-touts which have perfectly round seeds. 
Mendel himself showed that the mange-tout character, the soft 
constricted pod, was transferable. There are also mange-touts with 
fully wrinkled seeds and "grey" peas with small seeds (see Vilmorin- 
Andrieux, Plantes Potageres, 1888). 

154 A Defence of Mendel's 

Weldon, stating they may be considered separately, writes 
as follows: — 

"Tschermak has crossed Graue Riesen with five races of 
P. sativum, and he finds that the form of the first hybrid seeds 
follows the female parent, so that if races of P. sativum with 
round smooth seeds be crossed with Graue Riesen (which has 
flattened, feebly wrinkled seeds) the hybrids will be round and 
smooth or flattened and wrinkled, as the P. sativum or the 
Graue Riesen is used as female parent"^. There is here a more 
complex phenomenon than at first sight appears ; because if the 
flowers of the first hybrid generation are self-fertilised, the 
resulting seeds of the second generation invariably resemble 
those of the Graue Riesen in shape, although in colour they 
follow Mendel's law of segregation!" 

From this account who would not infer that we have 
here some mystery which does not accord with the 
Mendelian principles? As a matter of fact the case is 
dominance in a perfectly obvious if distinct form. 

Graue Riesen, a large grey sugar-pea, the pois sans 
parchemin geant of the French seedsmen, has full-yellow 
cotyledons and a highly coloured seed-coat of varying tints. 
In shape the seed is somewhat flattened with irregular 
slight indentations, lightly wrinkled if the term be preferred. 
Tschermak speaks of it in his first paper as " Same flach, 
zusammengedruckt " — a flat, compressed seed ; in his second 
paper as '"''flaclie, oft sckwach gerunzelte Cotyledonen-form" 
or cotyledon-shape, flat, often feebly wrinkled, as Professor 
Weldon translates. 

First-crosses made from this variety, each with a differ- 
ent form of P. sativum, are stated on the authority of 
Tschermak's five cases, to follow exclusively the maternal 
seed-shape. From ^^schwach gerunzelte,^' "feebly wrinkled," 
Professor Weldon easily passes to " wrinkled," and tells us 

* Correns found a similar result. 

Principles of Heredity 155 

that according as a round sativum or the Graue Riesen is 
used as mother, the first-cross seeds "will be round and 
smooth or flattened and wrinkled." 

As a matter of fact, however, the seeds of Graue Riesen 
though slightly wrinkled do not belong to the " wrinkled " 
class; but if the classification "wrinkled" and "round" is 
to be extended to such peas at all, they belong to the round. 
Mendel is careful to state that his round class are " either 
spherical or roundish, the depressions on the surface, when 
there are any, always slight" ; while the "wrinkled" class 
are "irregularly angular, deeply wrinkled"^." 

On this description alone it would be very likely that 
Graue Riesen should fall into the round class, and as such 
it behaves in its crosses, being dominant over wrinkled 
(see Nos. 3 and 6, below). I can see that in this case 
Professor Weldon has been partly misled by expressions 
of Tschermak's, but the facts of the second generation 
should have aroused suspicion. Neither author notices 
that as all five varieties crossed by Tschermak with Graue 
Riesen were round, the possibilities are not exhausted. 
Had Tschermak tried a really wrinkled sativum with Graue 
Riesen he would have seen this obvious explanation. 

As some of my own few observations of first-crosses bear 
on this point I may quote them, imperfect though they are. 

I grew the purple-flowered sugar-pea '' Pois sans par- 
chemin geant a tres large cosse,'^ a soft-podded "mange- 
tout" pea, flowers and seed-coats coloured, from Vilmorin's, 
probably identical with Graue Riesen. 

1. One flower of this variety fertilised with Pois trh 
nain de Bretagne (very small seed; yellow cotyledons ; very 

* ^'Entweder kugelrund oder rundlich, die Einsenkungen, wenn 
welche an der Oherfinche vorkommen, inimer nur seicht, oder sie sind 
unregelmnssig kantig, tief runzlig (P. quadratum).^' 

156 A Defence of 3IendeVs 

round) gave seven seeds indistinguishable (in their coats) 
from those of the mother, save for a doubtful increase in 
purple pigmentation of coats. 

2. Fertilised by Laxtons Alpha (green ; wrinkled ; coats 
transparent), two flowers gave 11 seeds exactly as above, 
the purple being in this case clearly increased. 

In the following the purple sugar-pea ws^s father. 

3. Laxton's A Ipha (green ; wrinkled ; coats transparent) 
fertilised by the purple sugar-pea gave one pod of four 
seeds with yellow cotyledons and round form. 

4. Fillhashet (green ; smooth but squared ; coats 
green) fertilised by the purple sugar-pea gave one pod 
with six seeds, yellow cotyledons * ; Fillhasket size and 
shape ; but the normally green coat yellowed near the hilum 
by xenia. 

5. Express (" blue "-green cotyledons and transparent 
skins ; round) fertilised with purple sugar-^Qd, gave one 
pod with four seeds, yellow cotyledons, shape round, much 
as in Fillhasket. 

6. British Quee7i (yellow cotyledons, wrinkled, white 
coats) 9 X purple sugar-pea gave two pods with seven seeds, 
cotyledons yellow, coats tinged greenish (xenia ?), all round. 

So much for the ''Purple^' sugar-pea. 

I got similar results with Mange-tout Deharhieux. This 
is a soft-podded Mange-tout or sugar-pea, with white flowers, 
large, flattish, smooth seeds, scarcely dimpled ; yellow coty- 

* The colour is the peculiarly deep yellow of the "grey" mange- 

Principles of Heredity 157 

7. Deharhieux fertilised hj Serpette nain hlanc (yellow 
cotyledons ; wrinkled ; white skin ; dwarf) gave one pod 
with six seeds, size and shape of Deharhieux, with slight 

8. Deharhieux by nain de Bretagne (very small ; yellow 
cotyledons; very round) gave three pods, 12 seeds, all 
yellow cotyledons, of which two pods had eight seeds iden- 
tical in shape with Debarhieux, while the third had four 
seeds like Debarhieux but more dimpled. The reciprocal 
cross gave two seeds exactly like nain de Bretagne. 

But it may be objected that the shape of this large 
grey pea is very peculiar* ; and that it maintains its type 
remarkably when fertilised by many distinct varieties 
though its pollen effects little or no change in them ; for, 
so long as round varieties of sativum are used as mothers, 
this is true as we have seen. But when once it is under- 
stood that in Graue Biesen there is no question of wrinkling, 
seeing that the variety behaves as a round variety, the 
shape and especially the size of the seed must be treated 
as a maternal property. 

Why the distinction between the shape of Graue 
Biesen and that of ordinary round peas should be a matter 
of maternal physiology we do not know. The question is 
one for the botanical chemist. But there is evidently very 
considerable regularity, the seeds borne by the cross-breds 
exhibiting the form of the "grey" pea, which is then a 
dominant character as much as the seed-coat characters 

* It is certainly subject to considerable changes according to 
conditions. Those ripened in my garden are without exception much 
larger and flatter than Vilmorin's seeds (now two years old) from 
which they grew. The colour of the coats is also much duller. These 
changes are just what is to be expected from the English climate — 
taken with the fact that my sample of this variety was late sown. 

158 A Defence of Menders 

are. And that is what Tschermak's Graue Riesen crosses 
actually did, thereby exhibiting dominance in a very clear 
form. To interject these cases as a mystery without pointing 
out how easily they can be reconciled with the "law of 
dominance" may throw an unskilled reader into gratuitous 

Finally, since the wrinkled peas, Laxtons Alpha and 
British Queen, pollinated hy a large flat mange-tout, witness 
Nos. 3 and 6 above, became round in both cases where this 
experiment was made, we here merely see the usual domin- 
ance of the non- wrinkled character ; though of course if a 
roz^Tzc^-seeded mother be used there can be no departure 
from the maternal shape, as far as roundness is concerned. 

Correns' observations on the shapes of a "grey" pea 
crossed with a round shelling pea, also quoted by Professor 
Weldon as showing no dominance of roundness, are of 
course of the same nature as those just discussed. 

C. Evidence of Knight and Laxton. 

In the last two sections we have seen that in using 
peas of the "grey" class, i.e. with brown, red, or purplish 
coats, special phenomena are to be looked for, and also 
that in the case of large " indented " peas, the phenomena 
of size and shape may show some divergence from that 
simple form of the phenomenon of dominance seen when 
ordinary round and wrinkled are crossed. Here the fuller 
discussion of these phenomena must have been left to await 
further experiment, were it not that we have other evidence 
bearing on the same questions. 

The first is that of Knight's well-known experiments, 
long familiar but until now hopelessly mysterious. I have 
not space to quote the various interpretations which Knight 
and others have put upon them, but as the Mendelian 

Principles of Heredity 159 

principle at once gives a complete account of the whole, 
this is scarcely necessary, though the matter is full of 
historical interest. 

Crossing a white pea with a very large grey purple- 
flowered form Knight (21) found that the peas so produced 
"were not in any sensible degree different from those 
afforded by other plants of the same [white] variety ; 
owing, I imagine, to the external covering of the seed (as 
I have found in other plants) being furnished entirely by 
the female*." All grew very tallf, and had colours of 
male parent :j:. The seeds they produced were dark grey§. 

" I had frequent occasion to observe, in this plant [the 
hybrid], a stronger tendency to produce purple blossoms, 
and coloured seeds, than white ones ; for when I introduced 
the farina of a purple blossom into a white one, the whole 
of the seeds in the succeeding year became coloured [viz. 
DR X D giving DD and DK\ ; but, when I endeavoured 
to discharge this colour, by reversing the process, a part 
only of them afforded plants with white blossoms ; this 
part sometimes occupying one end of the pod, and being at 
times irregularly intermixed with those which, when sown, 
retained their colour " [viz. DR x R giving DR and RR^^ 
(draws conclusions, now obviously erroneous ||). 

In this account we have nothing not readily intelligible 
in the light of Mendel's hypothesis. 

The next evidence is supplied by an exceptionally 
complete record of a most valuable experiment made by 

* Thus avoiding the error of Seton, see p. 144. There is no xenia 
perhaps because the seed-coat of mother was a transparent coat. 

t As heterozygotes often do. 

X Dominance of the purple form. 

§ Dominance of the grey coat as a maternal character. 

II Sherwood's view {J. R. Hort. Soc. xxii. p. 252) that this was the 
origin of the "Wrinkled" pea, seems very dubious. 

160 A Defence of MendeVs 

Laxton*. The whole story is replete with interest, and as 
it not only carries us on somewhat beyond the point 
reached by Mendel, but furnishes an excellent illustration 
of how his principles may be applied, I give the whole 
account in Laxton's words, only altering the paragraphing 
for clearness, and adding a commentary. The paper ap- 
pears in Jour. Hort. Soc. N.S. iii. 1872, p. 10, and very 
slightly abbreviated in Jour, of Hart, xviii. 1870, p. 86. 
Some points in the same article do not specially relate to 
this section, but for simplicity I treat the whole together. 

It is not too much to say that two years ago the 
whole of this story would have been a maze of be- 
wildering confusion. There are still some points in it 
that we cannot fully comprehend, for the case is one of far 
more than ordinary complexity, but the general outlines 
are now clear. In attempting to elucidate the phenomena 
it will be remembered that there are no statistics (those 
given being inapplicable), and the several offspring are 
only imperfectly referred to the several classes of seeds. 
This being so, our rationale cannot hope to be complete. 
Laxton states that as the seeds of peas are liable to change 
colour with keeping, for this and other reasons he sent to 
the Society a part of the seeds resulting from his experi- 
ment before it was brought to a conclusion. 

" The seeds exhibited were derived from a single experiment. 
Amongst these seeds will be observed some of several remarkable 
colours, including black, violet, purple-streaked and spotted, 
maple, grey, greenish, white, and almost every intermediate tint, 
the varied colours being apparently produced on the outer coat 
or envelope of the cotyledons only. 

* It will be well known to all practical horticulturalists that 
Laxton, originally of Stamford, made and brought out a large number 
of the best known modern peas. The firm is now in Bedford. 

Principles of Heredity 161 

The peas were selected for their colours, &c., from the third 
year's sowing in 1869 of the produce of a cross in 1866 of the 
early round white-seeded and white-flowered garden variety 
" Ringleader," which is about 2|^ ft. in height, fertilised by the 
pollen of the common purple-flowered "maple" pea, which is 
taller than " Ringleader," and has slightly indented seeds. 
I effected impregnation by removing the anthers of the seed- 
bearer, and applying the pollen at an early stage. This cross 
produced a pod containing five romid white peas, exactly like 
the ordinary " Ringleader " seeds'^. 

In 1867 I sowed these seeds, and all five produced tall 
purple-flowered purplish-stemmed plants t, and the seeds, with 
few exceptions, had all maple or brownish-streaked envelopes 
of various shades ; the remainder had entirely violet or deep 
purple-coloured envelopes i : in shape the peas were partly in- 

* A round white ? x grey c? giving the usual result, round, " white " 
(yellow) seeds. 

t Tall heterozygotes, with normal dominance of purple flowers. 

X Here we see dominance of the pigmented seed-coat as a maternal 
character over white seed-coat. The colours of the seed-coats are 
described as essentially two : maple or brown-streaked, and violet, the 
latter being a small minority. As the sequel shows, the latter are 
heterozygotes, not breeding true. Now Mendel found, and the fact 
has been confirmed both by Correns and myself, that crossing a grey 
pea which is capable of producing purple leads to such production as 
a form of xenia. 

We have here therefore in the purple seeds the union of dissimilar 
gametes, with production of xenia. But as the brown-streaked seeds 
are also in part heterozygous, the splitting of a compound allelomorph 
has probably taken place, though without precise statistics and 
allotment of offspring among the several seeds the point is uncertain. 
The colour of seed-coats in " grey " peas and probably " maples " also 
is, as was stated on p. 150, sensitive to conditions, but the whole 
difference between "maples" and purple is too much to attribute 
safely to such irregularity. " Maple " is the word used to describe 
certain seed-coats which are pigmented with intricate brown mottliugs 
on a paler buff ground. In French they are perdrix. 

B. 11 

162 A Defence of MendeVs 

dented ; but a few were round* Some of the plants ripened off 
earlier than the "maple," which, in comparison with "Ring- 
leader," is a late variety ; and although the pods were in many 
instances partially abortive, the produce was very large t. 

In 1868 I sowed the peas of the preceding year's growth, and 
selected various plants for earliness, productiveness, &c. Some 
of the plants had light-coloured stems and leaves ; these all 
showed white flowers, and produced round white seeds |. Others 
had purple flowers, showed the purple on the stems and at the 
axils of the stipules, and produced seeds with maple, grey, 
purple-streaked, or mottled, and a few only, again, with violet- 
coloured envelopes. Some of the seeds were round, some partially 
indented §. The pods on each plant, in the majority of instances, 
contained peas of like characters ; but in a few cases the peas in 
the same pod varied slightly, and in some instances a pod or 
two on the same plant contained seeds all distinct from the 
remainder II . The white-flowered plants were generally dwarfish, 

* This is not, as it stands, explicable. It seems from this point 
and also from what follows that if the account is truly given, some 
of the plants may have been mosaic with segregation of characters in 
particular flowers ; but see subsequent note. 

t As, commonly, in heterozygotes when fertile. 

Ij: Eecessive in flower-colour, seed-coat colour, and in seed-shape 
as a maternal character : pure recessives as the sequel proved. 

§ These are then a mixture of pure dominants and cross-bred 
dominants, and are now inextricably confused. This time the round 
seeds may have been all on particular plants — showing recessive seed- 
shape as a maternal character. It seems just possible that this 
fact suggested the idea of "round" seeds on the coZoure^ plants in 
the last generation. Till that result is confirmed it should be 
regarded as very doubtful on the evidence. But we cannot at the 
present time be sure how much difference there was between these 
round seeds and the normal maples in point of shape ; and on the 
whole it seems most probable tbat the roundness was a mere fluctua- 
tion, such as commonly occurs among the peas with large indented 

II Is this really evidence of segregation of characters, the flower 

Prmcii^les of Heredity 163 

of about the height of " Kingleader " ; but the coloured-flowered 
sorts varied altogether as to height, period of ripening, and 
colour and shape of seed*. Those seeds with violet-coloured 
envelopes jDroduced nearly all maple- or parti-coloured seeds, 
and only here and there one with a violet-coloured envelope ; 
that colour, again, appeared only incidentally, and in a like 
degree in the produce of the maple-coloured seeds f. 

In 1869 the seeds of various selections of the previous year 
were again sown separately ; and the white-seeded peas again 
produced only plants with white flowers and round white seeds %. 
Some of the coloured seeds, which I had expected would produce 
pm'ple-flowered plants, produced plants with white flowers and 
round white seeds only§ ; the majority, however, brought plants 
with purple flowers and with seeds principally marked with 
purple or grey, the maple- or brown-streaked being in the 
minority Ij. On some of the purple-flowered plants were again 
a few pods with peas differing entirely from the remainder on 
the same plant. In some pods the seeds were all white, in 
others all black, and in a few, again, all violet IT ; but those plants 
which bore maple-coloured seeds seemed the most constant and 
fixed in character of the purple-flowered seedlings**, and the 
purplish and grey peas, being of intermediate characters, ap- 

being the unit? In any case the possibility makes the experiment 
well worth repeating, especially as Correns has seen a phenomenon 
conceivably similar, 

* Being a mixture of heterozygotes (probably involving several 
pairs of allelomorphs) and homozygotes. 

t This looks as if the violet colour was merely due to irregularity 
of xenia. 

% Pure recessives. 

§ Pure recessives in coats showing maternal dominant character. 

II Now recognized as pure homozygotes. 

IT This seems almost certainly segregation by flower-units, and is 
as yet inexplicable on any other hypothesis. Especially paradoxical 
is the presence of " white " seeds on these plants. The impression is 
scarcely resistible that some remarkable phenomenon of segregation 
was really seen here. 

** Being now homozygotes. 


164 A Defence of MendeVs 

peared to vary most"^. The violet-coloured seeds again produced 
almost invariably purplish, grey, or maple peas, the clear violet 
colour only now and then appearing, either wholly in one pod or 
on a single pea or two in a pod. All the seeds of the purple- 
flowered plants were again either round or only partially in- 
dented ; and the plants varied as to height and earliness. In 
no case, however, does there seem to have been an intermediate- 
coloured flower ; for although in some flowers I thought I found 
the purple of a lighter shade, I believe this was owing to light, 
temperature, or other circumstances, and applied equally to the 
parent maple. I have never noticed a single tinted white flower 
nor an indented white seed in either of the three years' produce. 
The whole produce of the third sowing consisted of seeds of the 
colours and in the approximate quantities in order as follows, — 
viz. : 1st, white, about half ; 2nd, purplish, grey, and violet 
(intermediate colours), about three-eighths; and, 3rd, maple, 
about one-eighth. 

From the above I gather that the white-flowered white- 
seeded pea is (if I may use the term) an original variety well 
fixed and distinct entirely from the maple, that the two do not 
thoroughly intermingle (for whenever the white flower crops out, 
the plant and its parts all appear to follow exactly the characters 
of the white pea), and that the maple is a cross-bred variety 
which has become somewhat permanent and would seem to 
include amongst its ancestors one or more bearing seeds either 
altogether or partly violet- or purple- colo ured ; for although 
this colour does not appear on the seed of the "maple," it is 
very potent in the variety, and appears in many parts of the 
plant and its offspring from cross-fertilised flowers, sometimes 
on the external surface or at the sutures of the pods of the 
latter, at others on the seeds and stems, and very frequently on 
the seeds; and whenever it shows itself on any part of the 
plant, the flowers are invariably purple. My deductions have 
been confirmed by intercrosses effected between the various 
white-, blue-, some singularly bright green-seeded peas which I 
have selected, and the maple- and purple-podded and the purple- 
flowered sugar peas, and by reversing those crosses. 
* Being heterozygotes exclusively. 

Pri7iciples of Heredity 165 

I have also deduced from my experiments, in accordance 
with the conclusions of the late Mr Knight and others, that the 
colours of the envelopes of the seeds of peas immediately 
resulting from a cross are never changed*. I find, however, 
that the colour and probably the substance of the cotyledons 
are sometimes, but not always, changed by the cross fertilisation 
of two different varieties ; and I do not agree with Mr Knight 
that the form and size of the seeds produced are unaltered t; 
for I have on more than one occasion observed that the coty- 
ledons in the seeds directly resulting from a cross of a blue 
wrinkled pea fertilised by the pollen of a white round variety 
have been of a greenish-white colour |, and the seeds nearly 
round § and larger or smaller according as there may have been 
a difference in the size of the seeds of the two varieties 1 1 . 

I have also noticed that a cross between a round white and 
a blue wrinkled pea will in the third and fourth generations 
(second and third years' produce) at times bring forth blue 
round, blue wrinkled, white round and white wrinkled peas in 
the same pods, that the white round seeds, when again sown, 
will produce only white round seeds, that the white wrinkled 
seeds will, up to the fourth or fifth generation, produce both 
blue and white wrinkled and round peas, that the blue round 
peas will produce blue wrinkled and round peas, but that the 
blue wrinkled peas will bear only blue wrinkled seeds IT. This 

* The nature of this mistake is now clear ; for as stated above 
xenia is only likely to occur when the maternal seed-coat is pigmented. 
The violet coats in this experiment are themselves cases of xenia. 

t Knight, it was seen, crossed round ? x indented s and conse- 
quently got no change of form. 

X Cotyledons seen through coat. 

§ Ordinary dominance of round. 

II This is an extraordinary statement to be given as a general 
truth. There are sometimes indications of this kind, but certainly 
the facts are not usually as here stated. 

^ If we were obliged to suppose that this is a matured conclusion 
based on detailed observation it would of course constitute the most 
serious "exception" yet recorded. But it is clear that the five 

166 A Defence of MendeVs 

would seem to indicate that the white round and the blue 
wrinkled peas are distinct varieties derived from ancestors 
respectively possessing one only of those marked qualities; and, 
in my opinion, the white round peas trace their origin to a 
dwarfish pea having white flowers and round white seeds, and 
the blue wrinkled varieties to a tall variety, having also white 
flowers but blue wrinkled seeds. It is also noticeable, that from 
a single cross between two different peas many hundreds of 
' varieties, not only like one or both parents and intermediate, 
but apparently differing from either, may be produced in the 

statements are not mutually consistent. We have dominance of 
round white in first cross. 

In tbe second generation blue wrinkled give only bhie wrinkled, 
and blue round give blue wrinkled and round, in accordance with 
general experience. But we are told that white round give only 
white round. This would be true of some white rounds, but not, 
according to general experience, of all. Lastly we are told wldte 
wrinkled give all four classes. If we had not been just told by 
Laxton that the first cross showed dominance of white round, and 
that blue wrinkled and blue round give the Mendelian result, I should 
hesitate in face of this positive statement, but as it is inconsistent with 
the rest of the story I think it is unquestionably an error of statement. 
The context, and the argument based on the maple crosses show 
clearly also what was in Laxton's mind. He plainly expected the 
characters of the original pure varieties to separate out according to 
their original combinations, and this expectation confused his 
memory and general impressions. This, at least, until any such 
result is got by a fresh observer, using strict methods, is the only 
acceptable account. 

Of the same nature is the statement given by the late Mr Masters 
to Darwin {Animals and Plants, i. p. 318) that blue round, white round, 
blue wrinkled, and white wrinkled, all reproduced all four sorts during 
successive years. Seeing that one sort would give all four, and two 
would give two kinds, without special counting such an impression 
might easily be produced. There are the further difficulties due to 
seed-coat colour, and the fact that the distinction between round and 
wrinkled may need some discrimination. The sorts are not named, 
and the case cannot be further tested. 

Principles of Heredity 167 

course of three or four years (the shortest time which I have 
ascertained it takes to attain the chmax of variation in the 
produce of cross-fertihsed peas, and until which time it would 
seem useless to expect a fixed seedling variety to be produced*), 
although a reversion to the characters of either parent, or of 
any one of the ancestors, may take place at an earlier period. 

These circumstances do not appear to have been known to 
Mr Knight, as he seems to have carried on his experiments by 
continuing to cross his seedlings in the year succeeding their 
production from a cross and treating the results as reliable; 
whereas it is probable that the results might have been materially 
affected by the disturbing causes then in existence arising from 
the previous cross fertilisation, and which, I consider, would, in 
all cases where either parent has not become fixed or permanent, 
lead to results positively perplexing and uncertain, and to varia- 
ations almost innumerable. 1 have again selected, and intend 
to sow, watch, and report ; but as the usual climax of variation 
is nearly reached in the recorded experiment, I do not anticipate 
much fm'ther deviation, except in height and period of ripening — 
characters which are always very unstable in the pea. There 
are also important botanical and other variations and changes 
occurring in cross-fertilised peas to which it is not my 
province here to; but in conclusion I may, perhaps, in 
furtherance of the objects of this paper, be permitted to inquire 
whether any light can, from these observations or other means, 
be thrown upon the origin of the cultivated kinds of peas, 
especially the " maple " variety, and also as to the source whence 
the violet and other colours which appear at intervals on the 
seeds and in the ofis})ring of cross-fertilised purple-flowered peas 
are derived." 

The reader who has closely followed the preceding 
passage will begin to appreciate the way in which the new 
principles help us to interpret these hitherto paradoxical 
phenomena. Even in this case, imperfectly recorded as it 
is, we can form a fairly clear idea of what was taking place. 

* See later. 

168 A Defence of Menders 

If the "round" seeds really occurred as a distinct class, on 
the heterozygotes as described, it is just possible that the 
fact may be of great use hereafter. 

We are still far from understanding maternal seed- 
form — and perhaps size — as a dominant character. So far, 
as Miss Saunders has pointed out to me, it appears to be 
correlated with a thick and coloured seed-coat. 

We have now seen the nature of Professor Weldon's 
collection of contradictory evidence concerning dominance 
in peas. He tells us: ''Enough has been said to show the 
grave discrepancy between the evidence afforded by Mendel's 
experiments and that obtained by observers equally trust- 

He proceeds to a discussion of the Telephone and 
Telegraph group and recites facts, which I do not doubt 
for a moment, showing that in this group of peas — which 
have unquestionably been more or less "blend" or "mosaic" 
forms from their beginning — the "laws of dominance and 
segregation " do not hold. Professor Weldon's collection 
of the facts relating to Telephone, &c. has distinct value, 
and it is the chief addition he makes to our knowledge 
of these phenomena. The merit however of this addition 
is diminished by the erroneous conclusion drawn from it, as 
will be shown hereafter. Meanwhile the reader who has 
studied what has been written above on the general questions 
of stability, "purity," and "universal" dominance, will easily 
be able to estimate the significance of these phenomena and 
their applicability to Mendel's hypotheses. 

Pi'inciples of Heredity 169 

D. Miscellaneous cases in other plants and' animals. 

Professor Weldon proceeds : 

" In order to emphasize the need that the ancestry of the 
parents, used in crossing, should be considered in discussing the 
results of a cross, it may be well to give one or two more ex- 
amples of fundamental inconsistency between dififerent competent 

The '' one or two " run to three, viz. Stocks (hoariness 
and colour) ; Datura (character of fruits and colour of 
flowers) ; and lastly colours of Rats and Mice. Each of 
these subjects, as it happens, has been referred to in the 
forthcoming paper by Miss Saunders and myself. Datura 
and Matthiola have been subjected to several years' experi- 
ment and I venture to refer the reader who desires to see 
whether the facts are or are not in accord with Mendel's 
expectation and how far there is "fundamental inconsist- 
ency " amongst them to a perusal of our work. 

But as Professor Weldon refers to some points that 
have not been explicitly dealt with there, it will be safer 
to make each clear as we proceed. 

1. Stocks {Matthiola). Professor Weldon quotes 
Correns' observation that glabrous Stocks crossed with 
hoary gave offspring all hoary, while Trevor Clarke thus 
obtained some hoary and some glabrous. As there are 
some twenty different sorts of Stocks* it is not surprising 
that different observers should have chanced on different 
materials and obtained different results. Miss Saunders 

* The number in Haage and Schmidt's list exceeds 200, counting 

170 A Defence of Menders 

has investigated laws of heredity in Stocks on a large 
scale and an account of her results is included in our 
forthcoming Report. Here it must suffice to say that the 
cross hoary ? x glabrous c^ always gave offspring all hoary 
except once : that the cross glabrous ? x hoary ^ of several 
types gave all hoary ; but the same cross using other 
hoary types did frequently give a mixture, some of the 
offspring being hoary, others glabrous. Professor Weldon 
might immediately decide that here was the hoped for 
phenomenon of "reversed" dominance, due to ancestry, 
but here again that hypothesis is excluded. For the 
glabrous (recessive) cross-breds were pu?^e, and produced 
on self-fertilisation glabrous plants only, being in fact, 
almost beyond question, "false hybrids" (see p. 34), a 
specific phenomenon which has nothing to do with the 
question of dominance. 

Professor Weldon next suggests that there is discrepancy 
between the observations as to flower-colour. He tells us 
that Correns found violet Stocks crossed with ''yellowish 
white" gave violet or shades of violet flaked together. 
According to Professor Weldon 

" On the other hand Nobbe crossed a number of varieties of 
M. annua in which the flowers were white, violet, carmine- 
coloured, crimson or dark blue. These were crossed in various 
ways, and before a cross was made the colour of each parent was 
matched by a mixture of dry powdered colours which was pre- 
served. In every case the hybrid flower was of an intermediate 
colour, which could be matched by mixing the powders which 
recorded the parental colours. The proportions in which the 
powders were mixed are not given in each [any] case, but it is 
clear that the colours blended*." 

* The original passage is in Landwirths. Versuchstationen, 1888, 
XXXV. [not xxxiv.], p. 151. 

Princiijles of Heredity 171 

On comparing Professor Weldon's version with the 
originals we find the missing explanations. Having served 
some apprenticeship to the breeding of Stocks, we, here, 
are perhaps in a better position to take the points, but 
it is to me perfectly inexplicable how in such a simple 
matter as this he can have gone wrong. 

Note then 

(1) That Nobbe does not specify which colours he 
crossed together, beyond the fact that vjhite was crossed 
with each fertile form. The crimson form {Karmoisinfarhe)^ 
being double to the point of sterility, was not used. There 
remain then, white, carmine, and two purples (violet, ''dark 
blue"). When ivhite was crossed with either of these, 
Nobbe says the colour becomes paler, whichever sort gave 
the pollen. Nobbe does not state that he crossed carmine 
with the purples. 

(2) Professor Weldon gives no qualification in his 
version. Nobbe however states that he found it very 
difficult to distinguish the result of crossing calamine with 
vjhite from that obtained by crossing dark blue or violet 
with ivhite"^, thereby nullifying Professor Weldon's state- 
ment that in every case the cross was a simple mixture of 
the parental colours — a proposition sufficiently disproved by 
Miss Saunders' elaborate experiments. 

(3) Lately the champion of the " importance of small 
variations," Professor Weldon now prefers to treat the 
distinctions between established varieties as negligible 

* "£« ist sogar sehr schwierig, einen Untersehied in der Farhe der 
Kreuzungsprodukte von Karmin und Weiss gegeniiber Dunkelblau oder 
Violett und Weiss zu erkennen.'' 

172 A Defence of MendeVs 

fluctuations instead of specific phenomena"^. Therefore 
when Correns using ^'yellowish white^' obtained one result 
and Nobbe using ''white'' obtained another, Professor 
Weldon hurries to the conclusion that the results are 
comparable and therefore contradictory. Correns however 
though calling his flowers gelhlich-weiss is careful to state 
that they are described by Haage and Schmidt (the seed- 
men) as '' schwefel-gelh'' or sulphur-yellow. The topics 
Professor Weldon treats are so numerous that we cannot 
fairly expect him to be personally acquainted with all ; 
still had he looked at Stocks before writing, or even at the 
literature relating to them, he would have easily seen that 
these yellow Stocks are a thoroughly distinct formf ; and 
in accordance with this fact it would be surprising if they 
had not a distinctive behaviour in their crosses. To use 
our own terminology their colour character depends almost 
certainly on a compound allelomorph. Consequently there 
is no evidence of contradiction in the results, and appeal to 
ancestry is as unnecessary as futile. 

2. Datura. As for the evidence on Datura, I must 
refer the reader again to the experiments set forth in our 

The phenomena obey the ordinary Mendelian rules with 
accuracy. There are (as almost always where discontinuous 

* See also the case of Buchsbaum, p. 146, which received similar 

+ One of the peculiarities of most double " sulphur " races is that 
the singles they throw are white. See Vilmorin, Fleurs de pleine 
Terre, 1866, p. 354, note. In Wien. III. Gartenztg. 1891, p. 74, 
mention is made of a new race with singles also "sulphur," cp. 
Gartenztg. 1884, p. 46. Messrs Haage and Schmidt have kindly 
written to me that this new race has the alleged property, but that 
six other yellow races (two distinct colours) throw their singles white. 

Principles of Heredity 173 

variation is concerned) occasional cases of "mosaics," a 
phenomenon which has nothing to do with "ancestry." 

3. Colours of Rats and Mice. Professor Weldon 
reserves his collection of evidence on this subject for the 
last. In it we reach an indisputable contribution to the 
discussion — a reference to Crampe's papers, which together 
constitute without doubt the best evidence yet published, 
respecting colour-heredity in an animal. So far as I have 
discovered, the only previous reference to these memoirs is 
that of Eitzema Bos*, who alludes to them in a consideration 
of the alleged deterioration due to in-breeding. 

Now Crampe through a long period of years made an 
exhaustive study of the peculiarities of the colour-forms of 
Rats, white, black, grey and their piebalds, as exhibited in 

Till the appearance of Professor Weldon' s article 
Crampe's work was unknown to me, and all students of 
Heredity owe him a debt for putting it into general 
circulation. My attention had however been called by 
Dr Correns to the interesting results obtained by von 
Guaita, experimenting with crosses originally made between 
albino mice and piebald Japanese waltzing mice. This 
paper also gives fall details of an elaborate investigation 
admirably carried out and recorded. 

In the light of modern knowledge both these two 
researches furnish material of the most convincing character 
demonstrating the Mendelian principles. It would be a use- 
ful task to go over the evidence they contain and rearrange 
it in illustration of the laws now perceived. To do this here 
is manifestly impossible, and it must suffice to point out 
that the albino is a simple recessive in both cases (the 

* Biol Cblt. XIV. 1894, p. 79. 

174 A Defence of MendeVs 

waltzing character in mice being also a recessive), and that 
the "wild grey" form is one of the commonest heterozygotes 
— there appearing, like the yellow cotyledon-colour of peas, 
in either of two capacities, i.e. as a pure form, or as the 
heterozygote form of one or more combinations"^. 

Professor Weldon refers to both Crampe and von 
Guaita, whose results show an essential harmony in the 
fact that both found albino an obvious recessive, pure 
almost without exception, while the coloured forms show 
various phenomena of dominance. Both found hetero- 
zygous colour-types. He then searches for something that 
looks like a contradiction. Of this there is no lack in the 
works of Johann von Fischer (11) — an authority of a very 
different character — whom he quotes in the following 
few words : 

" In both rats and mice von Fischer says that piebald rats 
crossed with albino varieties of their species, give piebald young 
if the father only is piebald, white young if the mother only is 

But this is doing small justice to the completeness of 
Johann von Fischer's statement, which is indeed a pro- 
position of much more amazing import. 

That investigator in fact began by a study of the cross 
between the albino Ferret and the Polecat, as a means of 
testing whether they were two species or merely varieties. 
The cross, he found, was in colour and form a blend of the 
parental types. Therefore, he declares, the Ferret and the 

* The various " contradictions " which Professor Weldon suggests 
exist between Crampe, von Guaita and Colladon can almost certainly 
be explained by this circumstance. For Professor Weldon " wild- 
coloured" mice, however produced, are "wild-coloured" mice and 
no more (see Introduction). 

Princi])les of Heredity 175 

Polecat are two distinct species, because, " as everybody 
ought to know," 

" The result of a ovss between albino and normal [of 
one species] is always a constant one, namely an offspring 
like the father at least in colour "^^'^ 

whereas in crosses (between species) this is 7iot the case. 

And again, after reciting that the Ferret-Polecat crosses 
gave intermediates, he states : 

" But all this is not the case in crosses between albinos and 
normal animals within the species, in which always and without 
any exception the young resemble the father in colour t." 

These are admirable illustrations of what is meant by 
a ^^unicersar' proposition. But von Fischer doesn't stop 
here. He proceeds to give a collection of evidence in proof 
of this truth which he says '' ought to be known to every- 
one." He has observed the fact in regard to albino mole, 
albino shrew {&orex araneus), melanic squirrel {Sciwus 
vulgaris), albino ground-squirrel (Ifypudaeus te?Testris), 
albino hamster, albino rats, albino mice, piebald (grey- 
and- white or black-and-white) mice and rats, partially 
albino sparrow, and we are even presented with two cases 
in Man. Xo single exception was known to von Fischer J. 

* "Das Eesultat einer Kreuzung zwischen Albino- uud Normal- 
form ist stets, also, constant, ein dem Vater mindestens in der 
Farbung gleiches Junge." This law is predicated for the case in 
which both parents belong to the same species, 

t " Dieses Alles ist aber nie der Fall bei Kreuzungen unter 
Leucismen und normalen Thieren innerhalb der Species, bei denen 
stets und ohne jede Ausnahme die Jungen in Farbung dem Vater 

X He even withdraws two cases of his own previously published, 
in which grey and albino mice were alleged to have given mixtures, 
saying that this result must have been due to the broods having 
been accidentally mixed by the servants in his absence. 

176 A Defence of MendeVs 

In his subsequent paper von Fischer declares that from 
matings of rats in which the mothers were grey and the 
fathers albino he bred 2017 pure albinos ; and from albino 
mothers and grey fathers 3830 normal greys. "Not a 
single individual varied in any respect, or was in any way 

With piebalds the same result is asserted, save that 
certain melanic forms appeared. Finally von Fischer 
repeats his laws already reached, giving them now in this 
form : that if the offspring of a cross show only the colour 
of the father, then the parents are varieties of one species ; 
hut if the colour of the offspring be intermediate or different 
from that of the father, then the parents belong to distinct 

The reader may have already gathered that we have 
here that bane of the advocate — the witness who" proves 
too much. But why does Professor Weldon confine von 
Fischer to the few modest words recited above ? That 
author has — so far as colour is concerned — a complete 
law of heredity supported by copious " observations." 
Why go further? 

Professor Weldon "brings forth these strong reasons" 
of the rats and mice with the introductory sentence : 

" Examples might easily be multiplied, but as before, I have 
chosen rather to cite a few cases which rest on excellent authority, 
than to quote examples which may be doubted. I would only 
add one case among animals, in which the evidence concerning 
the inheritance of colour is affected by the ancestry of the 
varieties used." 

So once again Professor Weldon suggests that his laws 
of ancestry will explain even the discrepancies between 
von Fischer on the one hand and Crampe and von Guaita 

Principles of Heredity 177 

on the other but he does not tell us how he proposes to 
apply them. 

In the cross between the albino and the grey von Fischer 
tells us that both colours appear in the offspring, but always, 
without exception or variation, that of the father only, in 
5847 individuals. 

Surely, the law of ancestry, if he had a moment's 
confidence in it, might rather have warned Professor 
Weldon that von Fischer's results were wrong somewhere, 
of which there cannot be any serious doubt. The precise 
source of error is not easy to specify, but probably careless- 
ness and strong preconception of the expected result were 
largely responsible, though von Fischer says he did all the 
recording most carefully himself. 

Such then is the evidence resting "on excellent 
authority" : may we some day be privileged to see the 
" examples which may be doubted " ? 

The case of mice, invoked by Professor Weldon, has 
also been referred to in our Report. Its extraordinary 
value as illustrating Mendel's principles and the beautiful 
way in which that case may lead on to extensions of those 
principles are also there set forth (see the present 
Introduction, p. 25). Most if not all of such "conflicting" 
evidence can be reconciled by the steady application of 
the Mendelian principle that the progeny will be constant 
when — and only when* — similar gametes meet in fertilisa- 
tion, apart from any question of the characters of the 
parent which produces those gametes. 

* Excluding *' false hybridisations." 


178 A Defence of Mendel's 

V. Professor Weldon's quotations from Laxton. 

In support of his conclusions Professor Weldon adduces 
two passages from Laxton, some of whose testimony we 
have just considered. This further evidence of Laxton 
is so important that I reproduce it in full. The first 
passage, published in 1866, is as follows : — 

" The results of experiments in crossing the Pea tend to show 
that the colom* of the immediate offspring or second generation 
sometimes follows that of the female parent, is sometimes 
intermediate between that and the male parent, and is sometimes 
distinct from both; and although at times it partakes of the 
colour of the male, it has not been ascertained by the experimenter 
ever to follow the exact colour of the male parent"^. In shape, 
the seed frequently has an intermediate character, but as often 
follows that of either parent. In the second generation, in a 
single pod, the result of a cross of Peas different in shape and 
colour, the seeds are sometimes all intermediate, sometimes 
represent either or both parents in shape or colour, and 
sometimes both colours and characters, with their intermediates, 
appear. The results also seem to show that the third generation 
or the immediate offspring of a cross, frequently varies from its 
parents in a limited manner — usually in one direction only, 
but that the fourth generation produces numerous and wider 
variations f ; the seed often reverting partly to the colour and 
character of its ancestors of the first generation, partly partaking 
of the various intermediate colours and characters, and partly 
sporting quite away from any of its ancestry." 

* This is of course on account of the maternal seed characters. 
Unless the coat-characters are treated separately from the cotyledon- 
characters Laxton's description is very accurate. Both this and the 
statements respecting the " shape " of the seeds, a term which as used 
by Laxton means much more than merely "wrinkled" and "smooth," 
are recognizably true as general statements. 

t Separation of hypallelomorphs. 

Principles of Heredity 179 

Here Professor Weldoii's quotation ceases. It is un- 
fortunate he did not read on into the very next sentence 
with which the paragraph conchides : — 

" These sports apj^ear to become 
fixed and permanent in the next and succeeding generations ; 
and the tendency to revert and sport thenceforth seems to 
become checked if not absohitely stopped*." 

Now if Professor Weldon instead of leaving off on the 
w^ord "ancestry" had noticed tliis passage, I think his article 
would never have been written. 

Laxton proceeds : — 

" The experiments also tend to show that the height of the 
plant is singularly influenced by crossing ; a cross between two 
dwarf peas, commonly producing some dwarf and some tall 
[? in the second generation]; but on the other hand, a cross 
between two tall peas does not exhibit a tendency to diminution 
in height. 

"Xo perceptible difference appears to result from reversing 
the parents; the influence of the pollen of each parent at the 
olimax or fourth generation producing similar results t." 

The significance of this latter testimony I will presently 

Professor Weldon next appeals to a later paper of 
Laxton's published in 1890. From it he quotes this passage : 

" By means, however, of cross-fertilisation alone, and unless it 
be followed by careful and continuous selection, the labours of 
the cross- breeder, instead of benefiting the gardener, may lead 
to utter confusion," 

* The combinations being exhausted. Perhaps Professor Weldon 
thought his authority was here lapsing into palpable nonsense ! 

t Laxton constantly refers to this conception of the "climax" of — 
as we now perceive — analytical variation and recombination. Many 
citations could be given respecting his views on this "climax" (cp. 
p. 167). 


180 A Defence of MendeVs 

Here again the reader would have gained had Professor 
Weldon, instead of leaving off at the comma, gone on to 
the end of the paragraph, which proceeds thus : — 

"because, as I have previously stated, 
the Pea under ordinary conditions is much given to sporting 
and reversion, for when two dissimilar old or fixed varieties 
have been cross-fertilised, three or four generations at least 
must, under the most favourable circumstances, elapse before 
the progeny will become fixed or settled ; and from one such 
cross I have no doubt that, by sowing every individual Pea 
produced during the three or four generations, hundreds of 
different varieties may be obtained ; but as might be expected, 
I have found that where the two varieties desired to be 
intercrossed are unfixed, confusion will become confounded"^, 
and the variations continue through many generations, the 
number at length being utterly incalculable." 

Professor Weldon declares that Laxton's "experience 
was altogether different from that of Mendel." The reader 
will bear in mind that when Laxton speaks of fixing a 
variety he is not thinking particularly of seed-characters, 
but of all the complex characters, fertility, size, flavour, 
season of maturity, hardiness, etc., which go to make a 
serviceable pea. Considered carefully, Laxton's testimony 
is so closely in accord with Mendelian expectation that 
I can imagine no chance description in non-Mendelian 
language more accurately stating the phenomena. 

Here we are told in unmistakable terms the breaking 
up of the original combination of characters on crossing, 
their re-arrangement, that at the fourth or fifth generation 
the possibilities of sporting [sub-division of compound 
allelomorphs and re-combinations of them ?] are exhausted, 
that there are then definite forms which if selected are 

* Further subdivision and recombination of hypallelomorphs. 

Princi27les of Heredity 181 

thenceforth fixed [produced by union of similar gametes ?] 
that it takes longer to select some forms [dominants?] 
than others [recessives ?], that there may be " mule " 
forms'* or forms which cannot be fixed at allt [produced 
by union of dissimilar gametes?]. 

But Laxton tells us more than this. He shows us that 
numbers of varieties may be obtained — hundreds — "in- 
calculable numbers." Here too if Professor "Weldon had 
followed Mendel with even moderate care he would have 
found the secret. For in dealing with the crosses of 
Phaseolus Mendel clearly forecasts the conception of 
compound characters themselves again consisting oj definite 
units, all of which may be separated and re-combined in 
the possible combinations, laying for us the foundation of 
the new science of Analytical Biology, 

How did Professor Weldon, after reading Mendel, fail 
to perceive these principles permeating Laxton's facts? 
Laxton must have seen the very things that Mendel saw, 
and had he with his other gifts combined that penetration 
which detects a great principle hidden in the thin mist of 
"exceptions," we should have been able to claim for him 
that honour which must ever be Mendel's in the history of 

When Laxton speaks of selection and the need for it, 
he means, what the raiser of new varieties almost always 
means, the selection of definite forms, not impalpable 
fluctuations. When he says that without selection there 
will be utter confusion, he means — to use Mendelian terms 

* For instance the tails produced by crossing dwarfs are such 
" mules." Tschermak found in certain cases distinct increase in 
height in such a case, though not always (p. 531). 

t "The remarkably fine but unfixable pea Evolution,''^ Laxton, 
p. 37. 

182 A Defence of Menders 

— that the plant which shows the desired combination of 
characters must be chosen and bred from, and that if this 
be not done the grower will have endless combinations 
mixed together in his stock. If however such a selection 
be made in the fourth or fifth generation the breeder may 
very possibly have got a fixed form — namely, one that will 
breed true*. On the other hand he may light on one 
that does not breed true, and in the latter case it may be 
that the particular type he has chosen is not represented 
in the gametes and will 7iever breed true, though selected 
to the end of time. Of all this Mendel has given us the 
simple and final account. 

At Messrs Sutton and Sons, to whom I am most 
grateful for unlimited opportunities of study, I have seen 
exactly such a case as this. For many years Messrs Sutton 
have been engaged in developing new strains of the Chinese 
Primrose (Primula sinensis, hort.). Some thirty thoroughly 
distinct and striking varieties (not counting the Stellata 
or "Star" section) have already been produced which 
breed true or very nearly so. In 1899 Messrs Sutton 
called my attention to a strain knowTi as "Giant Lavender," 
a particularly fine form with pale magenta or lavender 
flowers, telling me that it had neVer become fixed. On 
examination it appeared that self-fertilised seed saved from 
this variety gave some magenta-reds, some lavenders, and 
some which are white on opening but tinge with very faint 
pink as the flower matures. 

On counting these three forms in two successive years 
the following figures appeared. Two separately bred 
batches raised from "Giant Lavender" were counted in 
each year. 

* Apart from fresh original variations, and perhaps in some cases 
imperfect homozygosis of some hypallelomorphs. 

Princij^les of Heredity 183 





faintly tinged 

1901 1st batch 




„ 2nd „ 




1902 1st „ 




„ 2nd „ 




54 96 45 

The numbers 54 : 96 : 45 approach the ratio 1:2:1 
so nearly that there can be no doubt we have here a simple 
case of Mendelian laws, operating without definite domi- 
nance, but rather with blending. 

When Laxton speaks of the "remarkably fine but 
unfixable pea Evolution^^ we now know for the first time 
exactly what the phenomenon meant. It, like the "Giant 
Lavender," was a "mule" form, not represented by germ- 
cells, and in each year arose by "self-crossing." 

This is only one case among many similar ones seen in 
the Chinese Primrose. In others there is no doubt that 
more complex factors are at work, the subdivision of 
compound characters, and so on. The history of the 
"Giant Lavender" goes back many years and is not 
known with sufficient precision for our purposes, but 
like all these forms it originated from crossings among 
the old simple colour varieties of sinensis. 

VI. The argument built on exceptions. 

So much for the enormous advance that the Mendelian 
principles already permit us to make. But what does 
Professor Weldon offer to substitute for all this ? Nothing. 

Professor Weldon suggests that a study of ancestry 
will help us. Having recited Tschermak's exceptions and 

184 A Defence of Menders 

the great irregularities seen in the Telephone group, he 
writes : 

"Taking these results together with Laxton's statements, 
and with the evidence afforded by the Telephone group of 
hybrids, I think we can only conclude that segregation of seed- 
characters is not of universal occurrence among cross-bred peas, 
and that when it does occur, it may or may not follow Mendel's 

Premising that when pure types are used the exceptions 
form but a small part of the whole, and that any supposed 
absence of "segregation" may have been variation, this 
statement is perfectly sound. He proceeds : — 

" The law of segregation, like the law of domi- 
nance, appears therefore to hold only for races of particular 
ancestry [my italics]. In special cases, other formulae expressing 
segregation have been offered, especially by De Vries and by 
Tschermak for other plants, but these seem as little likely to 
prove generally valid as Mendel's formula itself. 

"The fundamental mistake which vitiates all work based 
upon Mendel's method is the neglect of ancestry, and the 
attempt to regard the whole effect upon offspring, produced by 
a particular parent, as due to the existence in the parent of 
particular structural characters ; while the contradictory results 
obtained by those who have observed the offspring of parents 
identical in certain characters show clearly enough that not 
only the parents themselves, but their race, that is their ancestry, 
must be taken into account before the result of pairing them can 
be predicted." 

In this passage the Mendelian view is none too precisely 
represented. I should rather have said that it was from 
Mendel, first of all men, that we have learnt not to regard 
the effects produced on offspring " as due to the existence 
in the parent of particular structural characters." We 
have come rather to disregard the particular structure of 

Princiijles of Heredity 185 

the parent except in so far as it may give us a guide as to 
the nature of its gametes. 

This indication, if taken in the positive sense — as was 
sufficiently shown in considering the significance of the 
" mule " form or " hybrid-character " — we now know may 
be absolutely wortliless, and in any unfamiliar case is very 
likely to be so. Mendel has proved that the inheritance 
from individuals of identical ancestry may be entirely 
different : that from identical ancestry, without new 
variation, may be produced three kinds of individuals 
(in respect of each pair of characters), namely, individuals 
capable of transmitting one type, or another type, or both : 
moreover that the statistical relations of these three classes 
of individuals to each other will in a great number of cases 
be a definite one : and of all this he shows a complete 

Professor Weldon cannot deal with any part of this 
phenomenon. He does little more than allude to it in 
passing and point out exceptional cases. These he suggests 
a study of ancestry will explain. 

As a matter of fact a study of ancestry will give little 
guide — perhaps none — even as to the probability of the 
phenomenon of dominance of a character, none as to the 
probability of normal "purity" of germ-cells. Still less 
will it help to account for fluctuations in dominance, or 
irregularities in "purity." 

Ancestry and Dominance. 

In a series of astonishing paragraphs (pp. 241-2) Professor 
Weldon rises by gradual steps, from the exceptional facts 
regarding occasional dominance of green colour in Telephone 
to suggest that the whole phenomenon of dominance may he 

186 A Defence of MendeVs 

attributable to ancestry, and that in fact one character has no 
natural dominance over another, apart from what has been 
created by selection of ancestry. This piece of reasoning, 
one of the most remarkable examples of special pleading to 
be met with in scientific literature, must be read as a whole. 
I reproduce it entire, that the reader may appreciate this 
curious effort. The remarks between round parenthetical 
marks are Professor Weldon's, those between crotchets are 

" Mendel treats such characters as yellowness of cotyledons 
and the like as if the condition of the character in two given 
parents determined its condition in all their subsequent off- 
spring*. Now it is well known to breeders, and is clearly shown 
in a number of cases by Galton and Pearson, that the condition 
of an animal does not as a rule depend upon the condition of any 
one pair of ancestors alone, but in varying degrees upon the 
condition of all its ancestors in every past generation, the 
condition in each of the half-dozen nearest generations having 
a quite sensible effect. Mendel does not take the effect of 
differences of ancestry into account, but considers that any 
yellow-seeded pea, crossed with any green-seeded pea, will behave 
in a certain definite way, whatever the ancestry of the green and 
yellow peas may have been. (He does not say this in words, 
but his attempt to treat his results as generally true of the 
characters observed is unintelligible unless this hypothesis be 
assumed.) The experiments afford no evidence which can be 
held to justify this hypothesis. His observations on cotyledon 
colour, for example, are based upon 58 cross-fertilised flowers, 
all of which were borne upon ten plants ; and we are not even 
told whether these ten plants included individuals from more 
than two races. 

" The many thousands of individuals raised from these ten 

* Mendel, on the contrary, disregards the " condition of the 
character " in the parent altogether ; but is solely concerned with the 
nature of the characters of the gametes. 

Principles of Heredity 187 

plants afford an admirable illustration of the effect produced 
by crossing a few pairs of plants of known ancestry ; but while 
they show this perhaps better than any similar experiment, 
they do not afford the data necessary for a statement as to the 
behaviour of yellow-seeded peas in general, whatever their 
ancestry, when crossed with green-seeded peas of any ancestry. 
[Mendel of course makes no such statement.] 

" When this is remembered, the importance of the exceptions 
to dominance of yellow cotyledon-colour, or of smooth and 
rounded shape of seeds, observed by Tschermak, is much in- 
creased; because although they form a small percentage of his 
whole result, they form a very large percentage of the results 
obtained with peas of certain races. [Certainly.] The fact that 
Telephone behaved in crossing on the whole like a green-seeded 
race of exceptional dominance shows that something other than 
the mere character of the parental generation operated in this case. 
Thus in eight out of 27 seeds from the yellow Pois cP Auvergne 
$ X Telephone ^ the cotyledons were yellow with green patches ; 
the reciprocal cross gave two green and one yellow-and-green 
seed out of the whole ten obtained ; and the cross Telephone $ 
X (yellow-seeded) Buchshaum^ ^ gave on one occasion two green 
and four yellow seeds. 

"So the cross Couturier (orange-yellow) $ x the green-seeded 
Express ^ gave a number of seeds intermediate in colour. (It 
is not clear from Tschermak's paper whether all the seeds were 
of this colour, but certainly some of them were.) The green 
Plein le Panier [Fillbasket] $ x Couturier (^ in three crosses 
always gave either seeds of colour intermediate between green 
and yellow, or some yellow and some green seeds in the same 
pod. The cross reciprocal to this was not made ; but Express $ 
X Couturier ^ gave 22 seeds of which four were yellowish 
green t. 

"These facts show first that Mendel's law of dominance 
conspicuously fails for crosses between certain races, while it 

* Regarding this "exception" see p. 146. 
t See p. 148. 

188 A Defence of Mendel's 

appears to hold for others ; and secondly that the intensity of a 
character in one generation of a race is no trustworthy measure 
of its dominance in hybrids. The obvious suggestion is that the 
behaviour of an individual when crossed depends largely upon 
the characters of its ancestors'^. When it is remembered that 
peas are normally self-fertilised, and that more than one named 
variety may be selected out of the seeds of a single hybrid pod, 
it is seen to be probable that Mendel worked with a very definite 
combination of ancestral characters, and had no proper basis for 
generalisation about yellow and green peas of any ancestry" 
[which he never made]. 

Let us pause a moment before proceeding to the climax. 
Let the reader note we have been told of two groups of 
cases in which dominance of yellow failed or was ir- 
regular. (Why are not Gartner's and Seton's " exceptions " 
referred to here ?) In one of these groups Couturier was 
always one parent, either father or mother, and were it 
not for Tschermak's own obvious hesitation in regard to 
his own exceptions (see p. 148), I would gladly believe 
that Couturier — a form I do not know — may be an ex- 
ceptional variety. How Professor Weldon proposes to 
explain its peculiarities by reference to ancestry he omits 
to tell us. The Buchshaum case is already disposed of, 
for on Tschermak's showing, it is an unstable form. 

Happily, thanks to Professor Weldon, we know rather 
more of the third case, that of Telephone, which, whether 
as father or mother, w^as frequently found by Tschermak to 
give either green, greenish, or patchwork-seeds when crossed 
with yellow varieties. It behaves, in short, "like a green- 
seeded pea of exceptional dominance," as we are now told. 
For this dominant quality of Telephone's greenness we are 
asked to account hy appeal to its ancestry. May we not 

* Where was that "logician," the ** consulting-partner," when 
this piece of reasoning passed the firm? 

Principles of Heredity 189 

expect, then, this Telephone to be — if not a pure-bred green 
pea from time immemorial — at least as pure-bred as other 
green peas which do not exhibit dominance of green at all ? 
Now, what is Telephone ? Do not let us ask too much. 
Ancestry takes a lot of proving. We would not reject him 
^^ parce qu^il navait que soiccante d: onze quartiers^ <£- que le 
reste de son arhre genealogique avait ete perdu par V injure 
du tems.^' 

But with stupefaction we learn from Professor Weldon 
himself that Telephone is the very variety which he takes 
as his type of a permanent and incorrigible mongrel, a 
character it thoroughly deserves. 

From Telephone he made his colour scale I Tschermak 
declares the cotyledons to be "yellowish or whitish green, 
often entirely bright yellow*." So little is it a thorough- 
bred green pea, that it cannot always keep its own self- 
fertilised offspring green. Not only is this pea a parti- 
coloured mongrel, but Professor "Weldon himself quotes 
Culverwell that as late as 1882 both Telegraph and 
Telephone '^ will always come from one sort, more especially 
from the gTeen variety " ; and again regarding a supposed 
good sample of Telegraph that " Strange to say, although 
the peas were taken from one lot, those sown in January 
produced a great proportion of the light variety known as 
Telephone. These were of every shade of light green up to 
white, and could have been shown for either variety," Gard. 
Ohron. 1882 (2), p. 150. This is the variety whose green, 
it is suggested, partially *' dominates " over the yellow of 
Pois d' Auvergne, a yellow variety which has a clear lineage 
of about a century, and probably more. If, therefore, the 
facts regarding Telephone have any bearing on the signi- 

* " Speichergeivehe gelhlich — oder loeisslich — <7/vV», manclimal auch 
volUtdndig helUjelh.^' Tschermak (36), p. 480. 

190 A Defence of ^lendeVs 

ficance of ancestry, they point the opposite way from that 
in which Professor Welclon desires to proceed. 

In view of the evidence, the conclusion is forced upon 
me that the suggestion that "ancestry" may explain the 
facts regarding Telephone has no meaning behind it, but is 
merely a verbal obstacle. Two words more on Telephone. 
On p. 147 I ventured to hint that if we try to understand 
the nature of the appearance of green in the offspring of 
Telephone bred with yellow varieties, we are more likely to 
do so by comparing the facts with those of false hybridi- 
sation than with fluctuations in dominance. In this 
connection I would call the reader's attention to a point 
Professor Weldon misses, that Tschermak also got yelloivish- 
green seeds from Fillbashet {green) crossed with Telephone. 
I suggest therefore that Telephone's allelomorphs may be 
in part transmitted to its offspring in a state which needs 
no union with any corresponding allelomorph of the other 
gamete, just as may the allelomorphs of "false hybrids." 
It would be quite out of place here to pursue this reasoning, 
but the reader acquainted with special phenomena of 
heredity will probably be able fruitfully to extend it. 
It will be remembered that we have already seen the 
further fact that the behaviour of Telephone in respect to 
seed-shape was also peculiar (see p. 152). 

Whatever the future may decide on this interesting 
question it is evident that with Telephone (and possibly 
Buchsbauni) we are encountering a specific phenomenon, 
which calls for specific elucidation and not a case simply 
comparable with or contradicting the evidence of dominance 
in general. 

In this excursion we have seen something more of the 
" exceptions." Many have fallen, but some still stand, 
though even as to part of the remainder Tschermak enter- 

Principles of Heredity 191 

tains some doubts, and, it will be remembered, cautions his 
reader that of his exceptions some may be self-fertilisations, 
and some did not germinate*. Truly a slender basis to 
carry the coming structure ! 

But Professor Weldon cannot be warned. He told us 
the "law of dominance conspicuously fails for crosses 
between certain races." Thence the start. I venture to 
give the steps in this impetuous argument. There are 
exceptions t — a fair number if we count the bad ones — there 
may be more — must be more — are more — no doubt many 
more : so to the brink. Then the bold leap : may there 
not be as many cases one way as the other ? We have not 
tried half the sorts of Peas yet. There is still hope. 
True we know dominance of many characters in some 
hundreds of crosses, using some twenty varieties — not to 
speak of other plants and animals — but we do know some 
exceptions, of which a few are still good. So dominance 

"" In his latest publication on this subject, the notes to the 
■edition of Mendel in Ostwald's Klassiker (pp. 60 — 61), Tschermak, 
who has seen more true exceptions than any other observer, thus 
refers to them. As to dominance: — "Immerhin kommen vereinzelt 
nuch zweifellose Falle von Merkmalmischung , d. h. Ueherqangsformen 
zicischen gelber iind griiner Farhe, runder und runzeliger Form vor, 
die sich in iceiteren Generationen u'ie dominantmerhnalige Misolilinge 
verhalten." As to purity of the extracted recessives: — Ganz vereinzelt 
scheinen Ausnahmsfdlle vorzukommeny 

Kiister (22) also in a recent note on Mendelism points out, with 
reason, that the number of "exceptions" to dominance that Ave 
shall find, depends simply on the stringency with which the supposed 
*'law" is drawn. The same writer remarks further that Mendel 
makes no such rigid definition of dominance as his followers have 

t If the " logiciau-consulting-partner " will successfully apply this 
Fallacia acervalia, the " method of the vanishing heap," to dominant 
peas, he will need considerable leisure. 

192 A Defence of Mendel's 

may yet be all a myth, built up out of the petty facts those 
purblind experimenters chanced to gather. Let us take 
wider views. Let us look at fields more propitious — more 
what we would have them be ! Let us turn to eye-colour : 
at least there is no dominance in that. Thus Professor 
Weldon, telling us that Mendel "had no proper basis for 
generalisation about yellow and green peas of any ancestry," 
proceeds to this lamentable passage : — 

"Now in such a case of alternative inheritance as that of 
human eye-colour, it has been shown that a number of pairs of 
parents, one of whom has dark and the other blue eyes, will 
produce offspring of which nearly one half are dark-eyed, nearly 
one half are blue-eyed, a small but sensible percentage being 
children with mosaic eyes, the iris being a patch-work of 
lighter and darker portions. But the dark-eyed and light-eyed 
children are not equally distributed among all families; and it 
would almost certainly be possible, by selecting cases of marriage 
between men and women of appropriate ancestry, to demonstrate 
for their families a law of dominance of dark over light eye-colour, 
or of light over dark. Such a law might be as valid for the 
famines of selected ancestry as Mendel's laws are for his peas 
and for other peas of probably similar ancestral history, but it 
would fail when apphed to dark and light-eyed parents in 
general, — that is, to parents of any ancestry who happen to 
possess eyes of given colour." 

The suggestion amounts to this : that because there 
are exceptions to dominance in peas ; and because by some 
stupendous coincidence, or still more amazing incompetence, 
a bungler might have thought he found dominance of 
one eye-colour whereas really there was none*; therefore 

* I have no doubt there is no universal dominance in eye-colour. 
Is it quite certain there is no dominance at all? I have searched 
the works of Galton and Pearson relating to this subject without 
finding a clear proof. If there is in them material for this decision 

Principles of Heredity 193 

Professor TVeldon is at liberty to suggest there is a fair 
chance that Mendel and all who have followed him have 
either been the victims of this preposterous coincidence not 
once, but again and again ; or else persisted in the same 
egregious and perfectly gratuitous blunder. Professor 
Weldon is skilled in the Calculus of Chance : will he 
compute the probabilities in favour of his hypothesis ? 

Ancestri/ and purity of germ-cells. 

To what extent ancestry is likely to elucidate dominance 
we have now seen. We will briefly consider how laws 
derived from ancestry stand in regard to segregation of 
characters among the gametes. 

For Professor Weldon suggests that his view of ancestry 
will explain the facts not only in regard to dominance and 
its fluctuations but in regard to the purity of the germ-cells. 
He does not apply this suggestion in detail, for its error 
would be immediately exposed. In every strictly Mendelian 
case the ancestry of the pure extracted recessives or 
dominants, arising from the breeding of first crosses, is 
identical with that of the impure dominants [or impure 
recessives in cases where they exist]. Yet the posterity of 
each is wholly different. The pure extracted forms, in 
these simplest cases, are no more likely to produce the 
form with which they have been crossed than was their 
pure grandparent ; while the impure forms break up again 
into both grand-parental forms. 

Ancestry does not touch these facts in the least. They 

I may perhaps be pardoned for failing to discover it, since the tabula- 
tions are not prepared with this point in view. Eeference to the 
original records would soon clear up the point. 

B. 13 

194 A Defence of MendeVs 

and others like them have been a stumbling-block to all 
naturalists. Of such paradoxical phenomena Mendel now 
gives us the complete and final account. Will Professor 
Weldon indicate how he proposes to regard them? 

Let me here call the reader's particular attention to 
that section of Mendel's experiments to which Professor 
Weldon does not so much as allude. Not only did Mendel 
study the results of allowing his cross-breds (DE's) to 
fertilise themselves, giving the memorable ratio 

IBB : 2BE : IBR, 

but he fertilised those cross-breds (BE's) both with the 
pure dominant (B) and with the pure recessive (B) 
varieties reciprocally, obtaining in the former case the ratio 


and in the latter the ratio 


The BB group and the BB group thus produced giving 
on self-fertilisation pure B offspring and pure B offspring 
respectively, while the BB groups gave again 

IBB : 2BB : IBB. 

How does Professor Weldon propose to deal with these 
results, and by what reasoning can he suggest that 
considerations of ancestry are to be applied to them ? 
If I may venture to suggest what was in Mendel's mind 
when he applied this further test to his principles it 
was perhaps some such considerations as the following. 
Knowing that the cross-breds on self-fertilisation give 

IBB : 2BB : 1 BB 

three explanations are possible : 

Principles of Heredity 195 

(«) These cross-breds may produce pure D germs of 
both sexes and pure R germs of both sexes on an 
average in equal numbers. 

(h) Either the female, or the male, gametes may be 
alone differentiated according to the allelomorphs, 
into pure i)'s, pure i^'s, and crosses DR or RD, the 
gametes of the other sex being homogeneous and 
neutral in regard to those allelomorphs. 

(c) There may be some neutralisation or cancelling 
between characters in fertilisation occurring in such 
a way that the well-known ratios resulted. The 
absence of and inability to transmit the D character 
in the RR's, for instance, might have been due 
not to the original purity of the germs constituting 
them, but to some condition incidental to or connected 
with fertilisation. 

It is clear that Mendel realized (b) as a possibility, for 
he says DR was fertilised with the pure forms to test the 
composition of its egg-cells, but the reciprocal crosses were 
made to test the composition of the pollen of the hybrids. 
Readers familiar with the literature will know that both 
Gartner and Wichura had in many instances shown that 
the offspring of crosses in the form (axb)^ xc 6 were less 
variable than those of crosses in the form a ? x (6 x c) c^ , 
&c. This important fact in many cases is observed, and 
points to differentiation of characters occurring frequently 
among the male gametes when it does not occur or is much 
less marked among the maternal gametes. Mendel of 
course knew this, and proceeded to test for such a possi- 
bility, finding by the result that differentiation was the 
same in the gametes of both sexes*. 

* See Wichura (46), pp. 55-6. 


196 A Defence of MendeVs 

Of hypotheses (h) and (c) the results of recrossmg with 
the two pure forms dispose ; and we can suggest no 
hypothesis but {a) which gives an acceptable account of the 

It is the purity of the " extracted " recessives and the 
" extracted " dominants — primarily the former, as being 
easier to recognize — that constitutes the real proof of the 
validity of Mendel's principle. 

Using this principle we reach immediately results of 
the most far-reaching character. These theoretical de- 
ductions cannot be further treated here — but of the 
practical use of the principle a word may be said. Where- 
ever there is marked dominance of one character the 
breeder can at once get an indication of the amount of 
trouble he will have in getting his cross-bred true to either 
dominant or recessive character. He can only thus fore- 
cast the future of the race in regard to each such pair of 
characters taken severally, but this is an immeasurable 
advance on anything we knew before. More than this, it 
is certain that in some cases he will be able to detect the 
"mule" or heterozygous forms by the statistical frequency 
of their occurrence or by their structure, especially when 
dominance is absent, and sometimes even in cases where 
there is distinct dominance. With peas, the practical 
seedsman cares, as it happens, little or nothing for those 
simple characters of seed-structure, &c. that Mendel dealt 
with. He is concerned with size, fertility, flavour, and 
numerous similar characters. It is to these that Laxton 
(invoked by Professor Weldon) primarily refers, when he 
speaks of the elaborate selections which are needed to fix 
his novelties. 

We may now point tentatively to the way in which 
some even of these complex cases may be elucidated by an 

Principles of Heredity 197 

extension of Mendel's principle, though we cannot forget 
that there are other undetected factors at work. 

The value of the appeal to Ancestry. 

But it may be said that Professor Weldon's appeal to 
ancestry calls for more specific treatment. When he 
suggests ancestry as "one great reason" for the different 
properties displayed by different races or individuals, and 
as providing an account of other special phenomena of 
heredity, he is perhaps not to be taken to mean any 
definite ancestry, known or h)rpothetical. He may, in 
fact, be using the term "ancestry" merely as a brief 
equivalent signifying the previous history of the race or 
individual in question. But if such a plea be put forward, 
the real utility and value of the appeal to ancestry is 
even less evident than before. 

Ancestry, as used in the method of Galton and Pearson, 
means a definite thing. The whole merit of that method 
lies in the fact that by it a definite accord could be proved 
to exist between the observed characters and behaviour 
of specified descendants and the ascertained composition 
of their pedigree. Professor Weldon in now attributing 
the observed peculiarities of Telephone &c. to conjectural 
peculiarities of pedigree — if this be his meaning — renounces 
all that had positive value in the reference to ancestry. 
His is simply an appeal to ignorance. The introduction of 
the word "ancestry" in this sense contributes nothing. 
The suggestion that ancestry might explain peculiarities 
means no more than "we do not know how peculiarities are 
to be explained." So Professor Weldon's phrase "peas of 
probably similar ancestral history*" means "peas probably 
* See above, p. 192. 

198 A Defence of MendeVs 

similar " ; when he speaks of Mendel having obtained his 
results with " a few pairs of plants of known ancestry"^," he 
means "a few pairs of known plants" and no more ; when 
he writes that "the law of segregation, like the law of 
dominance appears to hold only for races of particular 
ancestry!," the statement loses nothing if we write simply 
" for particular races." We all know — the Mendelian, best 
of all — that particular races and particular individuals 
may, even though indistinguishable by any other test, 
exhibit peculiarities in heredity. 

But though on analysis those introductions of the word 
"ancestry" are found to add nothing, yet we can feel that 
as used by Professor Weldon they are intended to mean a 
great deal. Though the appeal may be confessedly to 
ignorance, the suggestion is implied that if we did know 
the pedigrees of these various forms we should then have 
some real light on their present structure or their present 
behaviour in breeding. Unfortunately there is not the 
smallest ground for even this hope. 

As Professor Weldon himself tells us J, conclusions from 
pedigree must be based on the conditions of the several 
ancestors ; and even more categorically (p. 244), " The 
degree to which a parental character affects offspring depends 
not only upon its development in the individual parent , hut 
on its degree of development in the ancestors of that parent. ^^ 
[My italics.] Having rehearsed this profession of an older 
faith Professor Weldon proceeds to stultify it in his very 
next paragraph. For there he once again reminds us that 
Telephone, the mongrel pea of recent origin, which does not 
breed true to seed characters, has yet manifested the peculiar 
power of stamping the recessive characters on its cross-bred 

* See above, p. 187. f See above, p. 184. 

\ See above, p. 186. 

Principles of Heredity 199 

offspring, though pure and stable varieties that have 
exhibited the same characters in a high degree for 
generations have not that power. As we now know, the 
presence or absence of a character in a progenitor may be 
no indication whatever as to the probable presence of the 
character in the offspring ; for the characters of the latter 
depend on gametic and not on zygotic differentiation. 

The problem is of a different order of complexity from 
that w^hich Professor Weldon suggests, and facts like these 
justify the affirmation that if we could at this moment 
bring together the whole series of individuals forming the 
pedigree of Telephone, or of any other plant or animal 
known to be aberrant as regards heredity, we should have 
no more knowledge of the nature of these aberrations ; no 
more prescience of the moment at which they would begin, 
or of their probable modes of manifestation ; no more 
criterion in fact as to the behaviour such an individual 
would exhibit in crossing*, or solid ground from which to 
forecast its posterity, than we have already. We should 
learn then — what we know already — that at some parti- 
cular point of time its peculiar constitution was created, 
and that its peculiar properties then manifested themselves : 
how or why this came about, we should no more compre- 
hend with the full ancestral series before us, than we can 
in ignorance of the ancestry. Some cross-breds follow 
Mendelian segregation ; others do not. In some, palpable 
dominance appears ; in others it is absent. 

If there were no ancestry, there would be no posterity. 
But to answer the question why certain of the posterity 
depart from the rule which others follow, we must know, 
not the ancestry, but how it came about either that at a 

* Beyond an indication as to the homogeneity or "purity" of its 
gametes at a given time. 

200 A Defence of Menders 

certain moment a certain gamete divided from its fellows in 
a special and unwonted fashion ; or, though the words are 
in part tautological, the reason why the union of two par- 
ticular gametes in fertilisation took place in such a way that 
gametes having new specific properties resulted*. No one 
yet knows how to use the facts of ancestry for the elucida- 
tion of these questions, or how to get from them a truth 
more precise than that contained in the statement that a 
diversity of specific consequences (in heredity) may follow 
an apparently single specific disturbance. Rarely even can 
we see so much. The appeal to ancestry, as introduced by 
Professor Weldon, masks the difficulty he dare not face. 

In other words, it is the cause of variation we are here 
seeking. To attack that problem no one has yet shown the 
way. Knowledge of a different order is wanted for that 
task ; and a compilation of ancestry, valuable as the 
exercise may be, does not provide that particular kind 
of knowledge. 

Of course when once we have discovered by experiment 
that — say. Telephone — manifests a peculiar behaviour in 
heredity, we can perhaps make certain forecasts regarding 
it with fair correctness ; but that any given race or 
individual will behave in such a way, is a fact not 
deducible from its ancestry, for the simple reason that 
organisms of identical ancestry may behave in wholly 
distinct, though often definite, ways. 

It is ficom this hitherto hopeless paradox that Mendel 
has begun at last to deliver us. The appeal to ancestry is 
a substitution of darkness for light. 

* May there be a connection between the extraordinary fertility 
and success of the Telephone grou-p of peas, and the peculiar frequency 
of a blended or mosaic condition of their allelomorphs? The con- 
jecture may be wild, but it is not impossible that the two phenomena 
may be interdependent. 

Principles of Heredity 201 

VII. The question of absolute purity of germ-cells. 

But let us go back to the cases of defective "purity" 
aud consider how the laws of ancestry stand in regard to 
them. It appears from the facts almost certain that purity 
may sometimes be wanting in a character which elsewhere 
usually manifests it. 

Here we approach a question of greater theoretical 
consequence to the right apprehension of the part borne 
by Mendelian principles in the physiology of heredity. 
We have to consider the question whether the purity of 
the gametes in respect of one or other antagonistic character 
is or is likely to be in case of any given character a 
universal truth ? The answer is unquestionably — No — ^but 
for reasons in which "ancestry" plays no part*. 

Hoping to interest English men of science in the 
Mendelian discoveries I offered in November 1900 a paper 
on this subject to " Nature." The article was of some 
length and exceeded the space that the Editor could grant 
without delay. I did not see my way to reduce it without 
injury to clearness, and consequently it was returned to 
me. At the time our own experiments were not ready for 
publication and it seemed that all I had to say would 
probably be common knowledge in the next few weeks, so 
no further attempt at publication was made. 

In that article I discussed this particular question of 
the absolute purity of the germ-cells, showing how, on 
the analogy of other bud-variations, it is almost certain 
that the germ-cells, even in respect to characters normally 
Mendelian, may on occasion present the same mixture of 
characters, whether apparently blended or mosaic, which 

* This discussion leaves "false hybridism" for separate con- 

202 A Defence of MendeVs 

we know so well elsewhere. Such a fact would in nowise 
diminish the importance of Mendel's discovery. The fact 
that mosaic peach-nectarines occur is no refutation of the 
fact that the total variation is common. Just as there 
may be trees with several such mosaic fruits, so there may 
be units, whether varieties, individual plants, flowers or 
gonads, or other structural units, bearing mosaic egg-cells 
or pollen grains. Nothing is more likely or more in 
accordance with analogy than that by selecting an in- 
dividual producing germs of blended or mosaic character, 
a race could be established continuing to produce such 
germs. Persistence of such blends or mosaics in asexual 
reproduction is well-known to horticulturists ; for example 
"bizarre" carnations, oranges streaked with " blood "- 
orange character, and many more. In the famous paper of 
Naudin, who came nearer to the discovery of the Mendelian 
principle than any other observer, a paper quoted by 
Professor Weldon, other examples are given. These forms, 
once obtained, can be multiplied hy division ; and there is 
no reason why a zygote formed by the union of mosaic or 
blended germs, once arisen, should not in the cell-divisions 
by which its gametes are formed, continue to divide in a 
similar manner and produce germs like those which united 
to form that zygote. The irregularity, once begun, may 
continue for an indefinite number of divisions. 

I am quite willing to suppose, with Professor Weldon 
(p. 248), that the pea Stratagem may, as he suggests, be 
such a case. I am even willing to accept provisionally as 
probable that when two gametes, themselves of mosaic or 
blended character, meet together in fertilisation, they are 
more likely to produce gametes of mosaic or blended 
character than of simply discontinuous character. Among 
Messrs Sutton's Primulas there are at least two striking 

Principles of Heredity 203 

cases of "flaked" or "bizarre" unions of bright colours 
and white which reproduce themselves by seed with fair 
constancy, though Mendelian purity in respect of these 
colours is elsewhere common in the varieties (I suspect 
mosaics of " false hybridism " among allelomorphs in some 
of these cases). Similarly Galton has shown that though 
children having one light-eyed and one dark-eyed parent 
generally have eyes either light or dark, the comparatively 
rare medium eye-coloured persons when they mate together 
frequently produce children with medium eye-colour. 

In this connection it may be worth while to allude to a 
point of some practical consequence. We know that when 
pure dominant — say yellow — is crossed with pure recessive 
— say green — the dominance of yellow is seen ; and we 
have every reason to believe this rule generally {not 
universally) true for pure varieties of peas. But we notice 
that in the case of a form like the pea, depending on 
human selection for its existence, it might be possible in 
a few years for the races with pure seed characters to be 
practically supplanted by the " mosaicized " races like the 
Telephone group, if the market found in these latter some 
specially serviceable quality. In the maincrop peas I 
suspect this very process is taking place*. After such a 

* Another practical point of the same nature arises from the great 
variability which these peas manifest in plant- as well as seed- 
characters. Mr Hurst of Burbage tells me that in e.g. William the 
First, a pea very variable in seed-characters also, tall plants may be 
so common that they have to be rogued out even when the variety is 
grqwn for the vegetable market, and that the same is true of several 
such varieties. It seems by no means improbable that it is by such 
I'oguing that the unstable mosaic or blend-form is preserved. In a 
thoroughly stable variety such as Ne Plus Ultra roguing is hardly 
necessary even for the seed-market. 

Mr N. N. Sherwood in his useful account of the origin and races 

204 A Defence of Menders 

revolution it might be possible for a future experimenter to 
conclude that Pismn sativum was by nature a "mosaicized" 
species in these respects, though the mosaic character may 
have arisen once in a seed or two as an exceptional 
phenomenon. When the same reasoning is extended to 
wild forms depending on other agencies for selection, some 
interesting conclusions may be reached. 

But in Mendelian cases we are concerned primarily not 
with the product of gametes of blended character, but with 
the consequences of the union of gametes already dis- 
continuously dissimilar. The existence of pure Mendelian 
gametes for given characters is perfectly compatible with 
the existence of blended or mosaic gametes for similar 
characters elsewhere, but this principle enables us to form 
a comprehensive and fruitful conception of the relation of 
the two phenomena to each other. As I also pointed 
out, through the imperfection of our method which does 
not yet permit us to see the differentiation among the 
gametes though we know it exists, we cannot yet as a 
rule obtain certain proof of the impurity of the gametes 
(except perhaps in the case of mosaics) as distinct from 
evidence of imperfect dominance. If however the case be 
one of a "mule" form, distinct from either parent, and 
not merely of dominance, there is no a priori reason why 
even this may not be possible ; for we should be able to 

of peas {Jour. R. Hort. Soc. xxii. 1899, p. 254) alludes to the great 
instability of this class of pea. To Laxton, he says, "we are indebted 
for a peculiar type of Pea, a round seed with a very slight indent, the 
first of this class sent out being William the First, the object being to 
get a very early blue- seeded indented Pea of the same earliness as the 
Sangster type with a blue seed, or in other words with a Wrinkled Pea 
flavour. This type of Pea is most difficult to keep true on account of 
the slight taint of the Wrinkled Pea in the breed, which causes it to 
run back to the Round variety." 

PHndijles of Heredity 205 

distinguish the results of breeding first crosses together 
into four classes : two pure forms, one or more blend or 
mosaic forms, and " mule " forms. Such a study could as 
yet only be attempted in simplest cases : for where we are 
concerned with a compound allelomorph capable of resolu- 
tion, the combinations of the integral components become 
so numerous as to make this finer classification practically 

But in many cases — perhaps a majority — though by 
Mendel's statistical method we can perceive the fluctuations 
in the numbers of the several products of fertilisation, we 
shall not know whether abnormalities in the distribution of 
those products are due to a decline in dominance, or to 
actual impurity of the gametes. We shall have further to 
consider, as afi'ecting the arithmetical results, the possibility 
of departure from the rule that each kind of gamete is 
produced in equal numbers* ; also that there may be 
the familiar difficulties in regard to possible selection and 
assortative matings among the gametes. 

I have now shown how the mosaic and blend-forms are 
to be regarded in the light of the Mendelian principle. 
What has Professor Weldon to say in reference to them ? 
His suggestion is definite enough — that a study of ancestry 
will explain the facts : how, we are not told. 

In speaking of the need of study of the characters of 
the race he is much nearer the mark, but when he adds 
"that is their ancestry," he goes wide again. When 
Telephone does not truly divide the antagonistic characters 
among its germ-cells this fact is in nowise simply traceable 
to its having originated in a cross — a history it shares with 
almost all the peas in the market — but to its own peculiar 

* In dealing with cases of decomposition or resolution of compound 
characters this consideration is of highest importance. 

206 A Defence of Menders 

nature. In such a case imperfect dominance need not 
surprise us. 

What we need in all these phenomena is a knowledge 
of the properties of each race, or variety, as we call it in 
peas. We must, as I have often pleaded, study the pro- 
perties of each form no otherwise than the chemist does the 
properties of his substances, and thus only can we hope to 
work our way through these phenomena. Ancestry holds 
no key to these facts ; for the same ancestry is common to 
own brothers and sisters endowed with dissimilar properties 
and producing dissimilar posterity. To the knowledge of 
the properties of each form and the laws which it obeys 
there are no short cuts. We have no periodic law to guide 
us. Each case must as yet be separately worked out. 

We can scarcely avoid mention of a further category of 
phenomena that are certain to be adduced in opposition to 
the general truth of the purity of the extracted forms. It 
is a fact well known to breeders that a highly-bred stock 
may, unless selections be continued, "degenerate." This 
has often been insisted on in regard to peas. I have been 
told of specific cases by Messrs Sutton and Sons, instances 
which could be multiplied. Surely, will reply the supporters 
of the theory of Ancestry, this is simply impurity in the 
extracted stocks manifesting itself at last. Such a con- 
clusion by no means follows, and the proof that it is 
inapplicable is obtained from the fact that the "degenera- 
tion," or variation as we should rather call it, need not 
lead to the production of any proximate ancestor of the 
selected stock at all, but immediately to a new form, or to 
one much more remote — in the case of some high class peas, 
e.g., to the form which Mr Sutton describes as "vetch- 
like," with short pods, and a very few small round seeds, 
two or three in a pod. Such plants are recognized by their 

Principles of Heredity 207 

appearance and are rigorously hoed out every year before 

To appreciate the meaning of these facts we must go 
back to what was said above on the nature of compound 
characters. We can perceive that, as Mendel showed, the 
integral characters of the varieties can be dissociated and 
re-combined in any combination. More than that; certain 
integral characters can be resolved into further integral 
components, by analytical variations. What is taking 
place in this process of resolution we cannot surmise, but 
we may liken the consequences of that process to various 
phenomena of analysis seen elsewhere. To continue the 
metaphor we may speak of return to the vetch-like type as 
a synthetical variation : well remembering that we know 
nothing of any substance being subtracted in the former 
case or added in the latter, and that the phenomenon is 
more likely to be primarily one of alteration in arrangement 
than in substance. 

A final proof that nothing is to be looked for from an 
appeal to ancestry is provided by the fact — of which the 
literature of variation contains numerous illustrations — 
that such newly synthesised forms, instead of themselves 
producing a large proportion of the high class variety which 
may have been their ancestor for a hundred generations, 
may produce almost nothing but individuals like themselves. 
A subject fraught with extraordinary interest will be the 
determination w^hether by crossing these newly synthesised 
forms with their parent, or another pure form, we may not 
succeed in reproducing a great part of the known series of 
components afresh. The pure parental form, produced, or 
extracted, by " analytical " breeding, would not in ordinary 
circumstances be capable of producing the other components 
from which it has been separated ; but by crossing it with 

208 A Defence of MendeVs Principles of Heredity 

the ''synthesised" variety it is not impossible that these 
components would again reappear. If this can be shown 
to be possible we shall have entirely new light on the nature 
of variation and stability. 


I trust what I have written has convinced the reader that 
we are, as was said in opening, at last beginning to move. 
Professor Weldon declares he has " no wish to belittle the 
importance of Mendel's achievement " ; he desires " simply 
to call attention to a series of facts which seem to him to 
suggest fruitful lines of inquiry." In this purpose I venture 
to assist him, for I am disposed to think that unaided he 
is — to borrow Horace Walpole's phrase — about as likely to 
light a fire with a wet dish-clout as to kindle interest in 
Mendel's discoveries by his tempered appreciation. If I 
have helped a little in this cause my time has not been 

In these pages I have only touched the edge of that new 
country which is stretching out before us, whence in ten 
years' time we shall look back on the present days of our 
captivity. Soon every science that deals with animals and 
plants will be teeming with discovery, made possible by 
Mendel's work. The breeder, whether of plants or of 
animals, no longer trudging in the old paths of tradition, 
will be second only to the chemist in resource and in 
foresight. Each conception of life in which heredity bears 
a part — and which of them is exempt? — must change before 
the coming rush of facts. 


1. CoRRENS, C. G. Mendel's Kegel iiber das Verhalten der 

Nachkommenscliaffc der Rassenbastarde, Ber. dent. hot. 
Ges., xviiL, 1900, p. 158. 

2. Gregor Mendel's " Versuche iiber Pflanzen-Hybriden " 

und die Bestatigung ihrer Ergebnisse durcb die neuesten 
Untersuchungen, Bot. Ztg., 1900, p. 229. 

3. Ueber Levkoyenbastarde zur Kenntniss der Grenzen 

der Mendel'schen Regeln, Bot. Chit., 1900, Vol. lxxxiv., 
p. 97. 

4. Bastarde zwischen Maisrassen, mit besonderer Beriick- 

sichtigung der Xenien, Bibliotheca Botanica, Hft. 5.3, 

5. Crampe. Kreuzungen zwischen Wanderratten verschiedener 

Farbe, Landwirths. Jahrh.., vi., 1877, p. 384. 

6. Zucht-Yersuche mit zabmen Wanderratten, 1. Re- 

sultate der Zucbt in Verwandtschaft, ihid., xii., 1883, 
p. 389. 2. Resultate der Kreiizung der zahmen Ratten 
niit wilden, ihid., xiii., 1884, p. 699. 

7. Die Gesetze der Vererbung der Farbe, ihid., xiv,, 

p. 539. 

8. Darwin, C. Variation of Animals and Plants under 

Domestication, ed. 2, i., pp. 348 and 428. 

9. Fischer, Johann von. Die Saugethiere dcs S' Petcrsburger 

Gouvernements, Zool. Garten, x., 1869, p. 336. 

10. litis {Mustela putorius) und Frett {Mustela furo), 

ihid., XIV., 1873, p. 108. 

B. 14 

210 Bibliograjjhy 

11. Fischer, Johann von. Beobachtungen iiber Kreuzungen 

verschiedener Farbenspielarten innerhalb einer Species, 
ihid., XV., 1874, p. 361. 

12. FocKE, W. 0. Die PJlanzen-MischUnge, Borntrager, Berlin, 


13. Ueber dichotype Gewachse. Oesterr. hot. Ztschr., 

XVIII., 1868, p. 139. 

14. Galton. F. Natural Inheritance, Macmillan and Co., 

London, 1889. - 

15. The Average Contribution of each several Ancestor 

to the total Heritage of the Offspring, Proc. Roy. Sac, 
LXL, 1897, p. 401. 

16. Gartner, C. F. von. Versuche und Beohachtungen uber 

die Bastarderzeugung im PJlanzenreich, Stuttgart, 

17. GiTAT, E, Ueber den directen Einfluss des Pollens auf 

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